Essentially reopening issue#2048, caching DrmSessions

Use case description

Need to cache DrmSession objects to avoid multiple license fetches for channel changes.

Proposed solution

The original solution was proposed in pull request: https://github.com/androidx/media/pull/2049

Alternatives considered

An alternative was suggested by @icbaker in #2048, however the solution needs tuning.

The suggestion was good, implement the cache in a custom DrmSessionManagerProvider. In order to force caching the DrmSession the balance of DrmSessionManager.prepare() to DrmSessionManager.release() needs to be controlled, issuing one additional prepare() call to preserve the DrmSession's in the session keep alive cache.

The only problem with the solution,

...
  @Override
  public DrmSessionManager get(MediaItem mediaItem) {
    DrmSessionManager drmSessionManager = delegate.get(mediaItem);
    if (cachedManagers.add(drmSessionManager)) {
      drmSessionManager.prepare();
    }
...

It is not possible to call prepare() from this method, it is called when the MediaSource is created, on the main application thread. The call needs to be on a player thread.

I'm sure there are ways to make this happen, the best choice for doing so is not clear to me.