Update MPV playback threading model (#481)

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

app/src/main/java/com/github/damontecres/wholphin/services/AppUpgradeHandler.kt

@@ -187,4 +187,14 @@ suspend fun upgradeApp(
             }
         }
     }
+
+    if (previous.isEqualOrBefore(Version.fromString("0.3.6-52-g0"))) {
+        if (Build.MODEL.equals("shield android tv", ignoreCase = true)) {
+            appPreferences.updateData {
+                it.updateMpvOptions {
+                    useGpuNext = false
+                }
+            }
+        }
+    }
 }