SDK / exoplayer

Some commands are run asynchronously and subsequent commands need
to wait until the previous one finished. This can be supported
by returning a Future for each command and using the existing
command execution logic to wait for each Future to complete.

As some MediaSessionStub code is now executed delayed to when it
was originally created, we also need to check if the session is
not released before triggering any actions or sending result codes.

Issue: androidx/media#85
PiperOrigin-RevId: 462101136
(cherry picked from commit 7cb7636e)