SDK / exoplayer

Although the underlying platform AudioTrack is capable of
writing the AV sync header from M onward, I've opted not
to use the functionality since it appears to allocate an
unnecessary (and large) number of ByteBuffers. We can swap
over from O if this is addressed.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=142138988