newPlayingPeriodHolder could be set then updated if seeking to a repeated period
that was loaded more than once. This led to MediaPeriodHolders leaking.
Only set newPlayingPeriodHolder once so that any later holders with the same
period identifier get released.
Also add a regression test. FakeMediaSource checks that all created
MediaPeriods were released when it is released.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172591937
1e79d6eb84