SDK / exoplayer

This wont help much until the buttons to get the dialogs
to display are focusable! Use of MediaController seems to
make that tricky (to be continued...).

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Created by MOE: https://github.com/google/moe
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When seeking to the default position in a period, the containing source may
actually return a position in another period. Multi-period live sources can do
this to seek the player to the live edge.

ExoPlayerImplInternal uses the same functionality when the playback position
reaches the end of a period to determine what period/position to play next.
This means that when playback transitions to a multi-period live source from
some other source (playing a concatenation of those two sources), the player
will play the live edge rather than the beginning of the earliest period.

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Created by MOE: https://github.com/google/moe
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The option was removed in
https://chromium.googlesource.com/webm/libvpx/+/67edc5e83be285004804c67c5348bbc81503a04d%5E%21/#F2.

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Created by MOE: https://github.com/google/moe
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Useful functionality promoted to core library:

1. Management of SurfaceHolder.Callback lifecycle
   promoted to SimpleExoPlayer
2. Ability to determine whether audio/video tracks
   exist but are all unsupported promoted to
   MappingTrackSelector.TrackInfo
3. Read external storage permissions check promoted
   to Util
4. SubtitleView given ability to act directly as a
   TextRenderer.Output to remove layer of indirection
5. SubtitleView given ability to configure itself to
   user's platform wide caption styling
6. KeyCompatibleMediaController promoted to library's
   UI package.

Relocation of boring stuff:

1. ID3 frame logging moved to EventLogger.

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Created by MOE: https://github.com/google/moe
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This change merges the duties of FormatEvaluator into
TrackSelection classes, so that both the static and
dynamic parts of track selection are implemented in a
single place.

New feature: Demo app now allows you to enable random
adaptation in the track selection dialog.

Notes:

- It should be quite easy to allow application side
track blacklisting in addition to source side, as an
extension to this. That would effectively allow
applications to do seamless/deferred track selection
by creating a TrackSelection with all tracks enabled,
and then toggling the blacklist flags to select the
ones they want to be active.

- It should be trivial to implement format blacklisting
for DASH and SS as an extension to this. Will do in a
follow up CL.

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Logic and some code is copied from V1.

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- Invoked --> Called
- Gets --> Returns
- "True if X. False otherwise." --> "Whether X."
- Removed some @returns where the Javadoc is in "Returns X" form

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This is in preparation for allowing MediaSources to skip over periods when one
period ends, which is needed for starting to play a multi-period live stream
that is concatenated on to another period at the live edge.

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- Suppress spurious resource type warnings.
- Make AS happy by making private method non-final.

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Created by MOE: https://github.com/google/moe
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As per the spec of MediaPeriod.getDurationUs, when a live
stream ends we need to start returning the correct final
duration.

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Created by MOE: https://github.com/google/moe
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We were forgetting to reset renderers during track
selections where muxed media is used.

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Created by MOE: https://github.com/google/moe
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It's only "needed" for HLS, and this requirement will
go away soon. It's safe to remove the flag ahead of
this because a TrackSelector can/should not attempt to
adapt between multiple audio/text tracks.

Also remove unnecessary restrictions on TrackGroups
being non-empty and only of known types.

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Created by MOE: https://github.com/google/moe
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- Remove null checks (never happens).
- Use "const".equals(Util.CONST) where possible.

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Created by MOE: https://github.com/google/moe
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The classes these tests are written for have changed
so much we'll have to write them again. We can refer
to V1 if we want to look at what the old tests did, so
there's no need to keep this file in the V2 tree at
the moment.

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Created by MOE: https://github.com/google/moe
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- TrackSelection now exposes the selected formats, ordered
  by decreasing bandwidth. This removes the need for DASH,
  SS and HLS to do the sorting individually.
- The change also removes the need to reconstruct TrackSelection
  instances with a different group index in various places
  (e.g. MergingMediaPeriod).
- This is also a step toward potentially packaging the
  FormatEvaluator inside of the TrackSelection (TBD).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128082468

According to the spec, there is no mandatory relation between segments'
sequence numbers of different variants. This CL ignores sequence numbers
when switching variants:

* In vod, the switching playback position is obtained by adding the
    duration of previous segments.
* In live playback this is not possible. It is assumed that the
    different live media playlists do not drift apart too much, so
    the playback position is obtained by subtracting the duration
    in reverse order.

In later CLs, the described mechanisms will become the fallback methods
by replacing them with the use of EXT-X-PROGRAM-DATE-TIME information
or more reliable sources.

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Created by MOE: https://github.com/google/moe
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ISSUE: #1681

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