AdsMediaSource posts AdsLoader.start to the main thread during preparation, but
the app may call AdsLoader.setPlayer(null) before it actually runs (e.g., if
initializing then quickly backgrounding the player).
This is valid usage of the API so handle this case instead of asserting. Because
not calling setPlayer at all is a pitfall of the API, track whether setPlayer
has been called and still assert that in AdsLoader.start.
PiperOrigin-RevId: 264329632
