These two methods are meant to indicate to the track selection that it's
started or stopped being used. This is helpful to schedule background tasks
related to track selection (e.g. register network change listeners etc.).
This intention is not clearly stated in the method docs.
Also, all track selections of all prebuffered periods stay enabled in
parallel at the moment. As the whole purpose of these methods is to know
whether dynamic updates via updateSelectedTrack may happen, it's better to
only enable track selections of the current loading media period. That's
similar to how we always forward the loading track selections to the
LoadControl.
This change:
1. Improves the JavaDoc of TrackSelection.
2. Disables track selections if loading moves to another period.
3. Reenables track selection if loading moves back to a previous period.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215199987
d97d289b6b