We currently try to call onAdPlaybackStarted even if the ad session
is not created yet and if not, we never call the callback afterwards.
Make sure to update and create the current session before trying to
send onAdPlaybackStarted.
As a result, we can merge updateSessions into the existing
handleTimelineChanged and handleDiscontinuity calls as they always
need to be called together.
PiperOrigin-RevId: 321383860
bb787d662d