SDK / exoplayer

Allow non-H264 adaptive decoders.

2. Common interface for manifest parsers.
- This effectively moves the common interface from the Fetcher level
  (i.e. ManifestFetcher) to the Parser level (i.e. ManifestParser).
- The motivation here is to allow the implementation of components that
  can work with a generic ManifestParser implementation.

- Skips unrecognized elements rather than crashing.
- FourCC treated as required for video and optional elsewhere,
  as per the SmoothStreaming spec.
- Only parse initData text when we're actually in the ProtectionHeader element

This means that after a decoder flush, the renderer will avoid
feeding non-keyframes into the decoder until it has received and
fed the first keyframe. The decoder has no way of correctly
decoding non-keyframes that arrive before a keyframe.

It looks like for the case of self-contained media segments,
it's possible to get stuck without failure in the case that
the load fails having loaded less than the length of the init
data.

Issue: #35

Issue: #31

Since we have a Format class as well, it's very confusing that
FormatHolder actually holds a MediaFormat. I think it's quite
likely that Format will need promoting into the root package as
part of the HLS work, which will make this even more confusing
(although it is possible that for HLS we'll define yet another
Format class, if it turns out we need significantly different
fields).

Note - I deliberately avoided renaming the formatHolder
args/params, because they're not particularly ambiguous and
because it introduces some ugly line breaks.

- Bring back requirement for the first video frame to be rendered
  before isReady returns true, *unless* we've deduced that the
  upstream source is serving multiple renderers.
- Ditto for requiring that the audio track has some buffered data.

- cache ref didn't work because it referred to a private variable
  (which isn't documented) from a public interface definition
  (which is). Meaning the Javadoc generator was trying to link
  to documentation that didn't exist.

The equals check we perform needs to ignore the max dimensions.
This tended to work in practice because formats would be the
same object, but in the case where different format objects
are used, things can break.

Issue: #4

- Add constants class. Currently housing a single lonely variable,
which is used generally throughout the library, and so no longer
nicely fits into a specific class.

- Rename a few other constants to add clear units.

- Made minor tweak to ExoPlayer documentation.

1. Use ints rather than longs.
2. Remove some counters that dont seem hugely useful.
3. Replace use of volatile with explicit method calls that
   cause a memory barrier. This is a lot more efficient than
   using volatile because it can be invoked only once per
   doSomeWork.

- Make MediaCodecTrackRenderer.isReady more permissive.
  This largely fixes #21
- Bring WebmExtractor closer to FragmentedMp4Extractor.
  The two will probably be placed under a common interface
  fairly soon, which will allow significant code
  deduplication.

* Remove concept of being prepared by simply reporting if format
and/or cues are known.
* Allow replacement of format and/or cues later in the stream.
* Initialization and index segments can be parsed independently
of one another but must be in order due to internal WebM dependencies.
* Let seekTo() work even when cues are unknown.