There's no user facing changes in this PR
This PR updates `MpvPlayer` to use a threading model more similar to
[ExoPlayer's
model](https://developer.android.com/reference/androidx/media3/exoplayer/ExoPlayer#threading-model).
There's now a dedicated thread looper message queue between the UI
interactions on the main thread and calls to `libmpv`. This is necessary
because previously, the main thread would make the calls into `libmpv`.
`libmpv` uses internal mutex locking for some operations which meant
that the main thread would be blocked momentarily.
Now, the UI issues a command (eg seek to X position) and the command is
added to the thread looper queue and processed on that separate thread.
This uses the standard Android Handler/Looper mechanisms.
Related to #235
Experimental MPV player backend
Related to #14, #22, & #85
You can switch to MPV in advanced settings and toggle using hardware
decoding or not.
This uses code and buildscripts from
https://github.com/mpv-android/mpv-android, plus other third party
libraries to build MPV
