SDK / exoplayer

This solves various issues around event association for buffering and
error throwing around period discontinuities.

The main changes are:
 - Logic around being "ready" at the end of a period no longer checks if the
   next period is prepared.
 - Advancing the playing period no longer checks if the next one is prepared.
 - Prepare errors are always thrown for the playing period.

This changes the semantics and assumptions about the "playing" period:
 1. The playing period can no longer assumed to be prepared.
 2. We no longer have a case where the queue is non-empty and the playing or
    reading periods are unassigned (=null).
Most other code changes ensure that these changed assumptions are handled.

Issue:#5407
PiperOrigin-RevId: 263776304