Ads can appear due to asynchronous ad tag requests completing after
earlier ads in a pod have loaded, so remove the requirement that the
ad count can't change. The MediaPeriodQueue should handling discarding
buffered content if an ad appears before already buffered content, so
I think this case is actually handled correctly by the core player
already.
Also remove the requirement that an ad URI can't change. This is a
defensive measure for now, but it's likely that a later fix in the IMA
SDK for an issue where loadAd is not called after preloading then
seeking before a preloaded ad plays will result in loadAd being called
more than once, and I think it's possible that the second call to
loadAd may have a different URI. Because the ad URI should only change
after an intermediate seek to another MediaPeriod, there shouldn't be
any problems with buffered data not getting discarded.
Issue: #7477
PiperOrigin-RevId: 316871371
