Allow changes in ad group count and order (to support pause-roll ads).

I wonder if there is a way of creating some custom media source with pause-roll support (pause-roll is an ad that is started when user resumes video from pause)?

AdsMediaSource does not support it and also is not accurate with delivering player states (https://github.com/google/ExoPlayer/issues/5209)

It seems that dynamic ConcatenatingMediaSource (combined from ClippingMediaSources with content and ad sources) might do the trick, but I'm not sure if there is a way to make player show the whole timebar for content instead of just timebar for current media source - is there a way?

Currently for post-roll I just save content position and load ad media and then reload content media back. But I would like to have that logic wrapped into custom media source to make my player code simpler.