Pause instead of close playback on KEYCODE_MEDIA_STOP (#1913)

Car head units send KEYCODE_MEDIA_STOP when powering off, which was
calling closePlayback() and destroying the session entirely. This removed
the media notification from the shade and required manually reopening the
app to resume listening.

Change to pause() so the session and notification stay alive, matching
the behavior of YouTube, Plex, and Jellyfin. When the car powers back on
and sends KEYCODE_MEDIA_PLAY, playback resumes in the same session.

android/app/src/main/java/com/audiobookshelf/app/player/MediaSessionCallback.kt

@@ -234,7 +234,7 @@ class MediaSessionCallback(var playerNotificationService:PlayerNotificationServi
             playerNotificationService.jumpBackward()
           }
           KeyEvent.KEYCODE_MEDIA_STOP -> {
-            playerNotificationService.closePlayback()
+            playerNotificationService.pause()
           }
           else -> {
             Log.d(tag, "KeyCode:${keyEvent.keyCode}")