This enables `AdsMediaSource` to be used with a live media
source that has a growing `AdPlaybackState` to which ad groups
can be appended.
Before this change, `AdsMediaSource` asserted that the number
of ad groups was kept the same, else an exception was thrown.
After this change, the assertion checks the validity of the
update and throws in case the update isn't considered valid.
An update is valid if ad groups are appended to the existing
`AdPlaybackState` or ads are appended to existing ad groups.
Further the `adGroupIndex` and `timeUs`of an existing ad
group can not be changed and once a media item is set for a
given ad, that media item can't be changed either.
PiperOrigin-RevId: 707244455
d4f4a2c1d4