SDK / exoplayer

(Preparation for Repeat Toggle Function - GitHub Issue #2577)

In addition, Timeline now also got a getPreviousWindowIndex and a
getNextPeriodIndex method with default implementations.

Changed ExoPlayerImplInternal and PlaybackControlView to use these
methods at all occurances of period and window index operations.

Note: Does not include repeat mode yet and no timelines are actually
using it so far. Please wait for the next CLs for this.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154520664