SDK / exoplayer

If a source is removed from the playlist, the player may still call createPeriod
for a period of the removed source as long as the new timeline hasn't been handled
by the player. These events are stale and can be ignored by using a dummy media
source. The stale media period will be released when the player handles the updated
timeline.

Issue:#4871

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214787090