SDK / exoplayer

This change makes all notification actions start MediaSessionService
in the background except COMMAND_PLAY which starts the service
in the foreground. This is to avoid ANRs that are raised if we don't
call MediaSessionService.startForeground() within 5 seconds since the
service was started in the foreground.

We only call MediaSessionService.startForeground() when
Player.getPlayWhenReady() returns true, and only COMMAND_PLAY sets
playWhenReady to true.

Issue: androidx/media#20

#minor-release

PiperOrigin-RevId: 433229604