SDK / exoplayer

Until recently, changing primary track formats were reported when the
corresponding media chunk was discarded which always happened immediately
after the sample has been read.

Now, media chunks may be discarded later on or in batches, leaving the
current reporting mechanism broken because changes may never be reported.

This fix separates the discarding from the reporting such that format changes
can be reported when the media chunk is first read from, while the discarding
operation only discards without reporting format changes.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176519071