If the app seeks after we get an ads manager but before the player exposes the
timeline with ads, we would end up expecting to play a preroll even after the
seek request arrived. This caused the player to get stuck.
Wait until a non-empty timeline has been exposed via onTimelineChanged before
initializing IMA (at which point it can start polling the player position). Seek
requests are not handled while an ad is playing.
PiperOrigin-RevId: 265058325
0085a7e761