feat(audio): Set audio context AudioContextConfigFocus.mixWithOthers by default (#3483)

# Description

AudioPlayers used to mix their sounds with other audio on the device,
which is not the proposed experience on Mobile devices. So this behavior
was changed in AudioPlayers, but it wasn't enforced due to a bug.
Obviously this behavior is not wanted for Games as multiple sounds
should play simultaneously, so this recovers the old functionality
again.

## Checklist
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- [x] I have followed the [Contributor Guide] when preparing my PR.
- [ ] I have updated/added tests for ALL new/updated/fixed
functionality.
- [ ] I have updated/added relevant documentation in `docs` and added
dartdoc comments with `///`.
- [x] I have updated/added relevant examples in `examples` or `docs`.

Not sure, if I can test this properly, as it's on a system level. I
could ensure the values are set though (?)

## Breaking Change?
I'm actually not sure, if this is breaking, I think it's not, as
`audioplayers` used to work like `mixWithOthers` due to a bug, but is
not anymore.

- [ ] Yes, this PR is a breaking change.
- [x] No, this PR is not a breaking change.

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---------

Co-authored-by: Lukas Klingsbo <me@lukas.fyi>

packages/flame_audio/example/lib/main.dart

@@ -39,9 +39,9 @@ class AudioGame extends FlameGame with TapDetector {
 
   Rect get button => Rect.fromLTWH(20, size.y - 300, size.x - 40, 200);
 
-  void startBgmMusic() {
-    FlameAudio.bgm.initialize();
-    FlameAudio.bgm.play('music/bg_music.ogg');
+  Future<void> startBgmMusic() async {
+    await FlameAudio.bgm.initialize();
+    await FlameAudio.bgm.play('music/bg_music.ogg');
   }
 
   void fireOne() {