SDK / exoplayer

Handling of this constant is already in place everywhere, this
just documents that its use is allowed.

Avoid using the VP8 decoder on Samsung Galaxy S4 Mini.
Issue: #548

Avoid using the MP3 decoder on Sony Xperia Z/API level 16.
Issue: #377

Issue: #585

Issue: #576

Prior to this change, there was a bug where playback would
fail with the following steps:

1. Start playback.
2. Pause playback.
3. Disable all renderers.
4. Enable at least one renderer.
5. Resume playback.

The new logic assumes that an input format change will be
followed by an output format change, but I think this is
pretty much guaranteed. If this weren't to happen then the
new pixel aspect ratio wont be picked up, but I think it's
extremely unlikely (it would require the format to stay
exactly the same except for the pixel aspect ratio, which
would be bizarre).

This makes it easier to add an extension for adjusting
audio playback rate.

Issue: #417

1. An optional U+FEFF BYTE ORDER MARK (BOM) character.
2. The string "WEBVTT".
3. Optionally, either a U+0020 SPACE character or a U+0009 CHARACTER
   TABULATION (tab) character followed by any number of characters that
   are not U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters.
4. Exactly one WebVTT line terminators to terminate the line with the file
   magic and separate it from the rest of the body.

Issue: #580

This is useful for reproducibility in playback tests.

Improve Mp4Extractor test.

Add support for Xiph lacing in Matroska files.

Add support for EBML lacing in Matroska files.

Handle the initial sticky intent for HDMI audio plug.

An accumulation of several fixes:

1. Change to HlsExtractorWrapper is just a move + documentating
   things that were already true + adding a precondition in the
   configureSpliceTo method.

2. Change in HlsSampleSource.readData ensures that configureSpliceTo
   and hasSamples aren't called on an extractor that isn't prepared.

3. The other change in HlsSampleSource ensures the correct "previous"
   TsChunk is used. If a TsChunk fails to load and is replaced, the
   previous chunk should be the one before that whose load completed
   successfully.

4. Determine switchingVariantSpliced based on the actual format of the
   previous chunk, so it's set correctly in the case of a TsChunk load
   failure and subsequent replacement.

This fixes the case where we need to switch from one variant to
another during preparation, having loaded zero samples, because
the first chunk for that variant gave us a 404 error. In this
case the first extractor will have no samples, and there will
be a second extractor that does.

There was a mess where we were indexing into both a list of variants
and a (differently ordered and possibly of differing length) list of
formats. This sanitises everything.

1. Refine the way TtmlNode handles whitespace collapsing when constructing
   the output text. Can of worms.
2. Start using SpannableStringBuilder. This will allow attaching of spans
   in getText as nodes are encountered, which is how we'll be incorporating
   styling information into the result.