SDK / exoplayer

While the timeline is empty, we keep a dummy MediaPeriodId in PlaybackInfo with
a period index of 0. We leak this MediaPeriodId in actual use in these
situations:
 1. When issuing an IllegalSeekPosition after preparation. The timeline becomes
    non-empty, but the media period id stays at its dummy value.
 2. When re-adding sources to a previously empty timeline. The dummy period id
    is used as the start position for the new non-empty timeline.

This change makes:
 - the constructor of PlaybackInfo using those dummy values more explicit
 - prevents the issues above by using the correct default position in the new
   non-empty timeline for the above mentioned cases.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205803006