SDK / exoplayer

This simplifies some position tracking needs for an app implementing
SimpleBasePlayer.
 - The period index can always be derived from the media item index
   and the position. So there is no need to set it separately.
 - The media item index can be left unset in the State in case the app
   doesn't care about the value or wants to set it the default start
   index (e.g. while the playlist is still empty where UNSET is
   different from zero).
 - Similarly, we should allow to set the content position (and buffered
   position) to C.TIME_UNSET to let the app ignore it or indicate the
   default position explictly.

PiperOrigin-RevId: 495352633
(cherry picked from commit 91557ac9)