SDK / exoplayer

When the renderer media clock source read its stream to end but is not ready,
this means one of two things. Either the next period is not prepared yet and
we need to stop the renderers and buffer until it's prepared, or we are
waiting for another track in the current period with a uneven (longer)
duration.

The second case was already covered by this if condition and uses the standalone
clock instead to continue.

The first case now also uses the standalone clock, but it doesn't make a
difference, because both clocks are stopped and still synchronized.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175134975