SDK / exoplayer

This will be needed for keeping WebVTT subtitle and separate
audio tracks is sync whilst loading.

Issue: #151

This OMX component is listed but can't be instantiated on this device.

According to the GitHub issue, some other devices are also affected, so these
will have to be added too.

Issue: #377

They're current location is annoying, because it creates a dependency
from the dash package to the webvtt package. For apps like Play Movies
where WEBVTT isn't used, it's nice just to delete the whole package at
import time, which requires that this dependency be removed.

HLS AAC fix

Issue: #479

Seems generally useful to have.

Issue: #473

Fixes issue where an NPE occurs when RollingSampleBuffer is created
with a null Allocator.

- There's definitely potential for more code sharing in these
classes, but deferring for now.
- Also made no-scheme default to file://, and allowed smoothstreaming
URLs to be specified with or without the /Manifest suffix.

Before preparation, and when seeking, Mp3Extractor did not handle retrying:
- synchronizedHeader was set before the header was known to be valid, which
  means that after seeing one valid frame header and then failing to read, the
  synchronization would be treated as complete.
- The input buffer would keep data loaded during synchronization but on the next
  call to synchronize when retrying it was not returned to the mark position to
  re-parse the data.

This change fixes these issues.

- Make HlsPlaylistParser treat non-positive dimensions as unknown.
- Make HlsPlaylistParser parse floating point resolutions, because
  technically that's how they're spec'd.
- Make VideoFormatSelectorUtil treat non-position dimensions as unknown.

Issue: #461

1. Make DefaultUriDataSource robust against exceptions through from close().
2. Make AssetDataSource handle file:///android_asset/ URIs.

When I moved the id3Reader instantiation out of the while
loop (below where it is now) it was no longer guarded by
the streamTypes.get(streamType) deduping check. This brings
back an equivalent check.

When a XING header is present but not usable (due to missing fields), CBR
seeking can be used instead. It relies on the bitrate. The bitrate from the
unusable XING header is not correct, which leads to incorrect seeking.

Also fix VBRI seeking by setting the correct offset on the frame to parse. Few
people seem to use that format, but I have found two very short truncated
samples which were falling back to the CBR case before but are using VBRI with
this change.