There is a race with the ad period preparation having completed
and `onDownstreamFormatChanged` being called when a live stream
is joined in an ad period. In this case the stream event metadata
of the period is immediately emitted and causing an ad media period
being created that is selected in `getMediaPeriodForEvent` before
being prepared (1 out of 4).
Using an `isPrepared` flag makes sure we don't hand out the media
period to early in `getMediaPeriodForEvent`.
PiperOrigin-RevId: 522340046
