Period transitions can either be to new windows (in which case the default
position for the new window should be loaded) or to the next period of the
current window (in which case the the new period should be played from zero).
Fix the logic for calculating the new period index to load to implement this.
In processManifest, periodsById may contain periods that have been removed from
the manifest, which are still being used by the player (it releases periods on
receiving the source info refresh after processManifest returns). Ignore
periods that have been removed from the manifest when calling updateManifest.
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