SDK / exoplayer

`prepare()` now logs a warning if it's called before `setPlayer()`
because it's not possible to tell if it's being called on the wrong
thread (since https://github.com/google/ExoPlayer/commit/9d028b33017e82b05a9de4e231355667b1b78264).

This change finds all the places one is called immediately after the
other and flips the order to be more correct.

Issue: androidx/media#350

#minor-release

PiperOrigin-RevId: 526582294
(cherry picked from commit aa4e0080)