SDK / exoplayer

In the same way it made sense to tie CacheKeyFactory and CacheDataSource
together in
https://github.com/google/ExoPlayer/commit/7ea83d7167d0b021fb280fca2b5dfa4a5616b5c1,
it makes sense to tie the PriorityTaskManager to the CacheDataSource as
well. This prevents error prone scenarios where one can end up being
passed around without (or with the wrong instance of) the other.

This change also internalizes use of PriorityDataSource directly into
CacheDataSource, rather than requiring the caller to chain things
themselves.

PiperOrigin-RevId: 307647290