Previously `MediaPeriodQueue` would return null if an ad media URI hadn't
loaded yet, but this meant that the player could be stuck in `STATE_READY` if
an `AdsLoader` unexpectedly didn't provide an ad URI. Fix this behavior by
masking ad media periods. `MaskingMediaPeriod` no longer requires a
`MediaSource` to instantiate it.
This also fixes a specific case where playback gets stuck when using the IMA
extension with an empty ad where the IMA SDK unexpectedly doesn't notify the ad
group fetch error.
Issue: #8205
PiperOrigin-RevId: 344984824
