SDK / exoplayer

I think it was just wrong that it was package private before, since
it resulted in our public API referencing something that's not part
of the public API:

https://google.github.io/ExoPlayer/doc/reference/com/google/android/exoplayer2/source/chunk/BaseMediaChunk.html#init-com.google.android.exoplayer2.source.chunk.BaseMediaChunkOutput-

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198396555