[Xam/XMP App] - Clean up & improvements

- Clean up xmp app
- XMPGetTitlePlaylistBufferSize now returns proper errors
- Implement XMPGetDashInItState
- Add missing xnotifications

src/xenia/apu/audio_media_player.h

@@ -77,6 +77,7 @@ class AudioMediaPlayer {
   }
 
   XmpApp::PlaybackClient GetPlaybackClient() const { return playback_client_; }
+  uint32_t GetDashInItState() const { return dash_init_state; }
 
   bool IsTitleInPlaybackControl() const {
     return playback_client_ == XmpApp::PlaybackClient::kTitle ||
@@ -106,6 +107,7 @@ class AudioMediaPlayer {
   XmpApp::RepeatMode repeat_mode_ = XmpApp::RepeatMode::kPlaylist;
   XmpApp::PlaybackFlags playback_flags_ = XmpApp::PlaybackFlags::kDefault;
   float volume_ = 1.0f;
+  uint32_t dash_init_state = 0;
 
   std::unordered_map<uint32_t, std::unique_ptr<XmpApp::Playlist>> playlists_;
   XmpApp::Playlist* active_playlist_;