playback/player: android: Unref player in native_free()

This makes sure it is actually shut down and does not emit any signals anymore
later, and also prevents a memory leak.

playback/player/android/app/src/main/jni/player.c

@@ -224,6 +224,7 @@ native_free (JNIEnv * env, jobject thiz)
   if (!player)
     return;
 
+  g_object_unref (player->player);
   (*env)->DeleteGlobalRef (env, player->java_player);
   g_free (player);
   SET_CUSTOM_DATA (env, thiz, native_player_field_id, NULL);