`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
