-Designated background player as default media button receiver.

-Fixed media button intent causing illegal state exception when sent from external apps.

app/src/main/AndroidManifest.xml

@@ -42,7 +42,11 @@
 
         <service
             android:name=".player.BackgroundPlayer"
-            android:exported="false"/>
+            android:exported="false">
+            <intent-filter>
+                <action android:name="android.intent.action.MEDIA_BUTTON" />
+            </intent-filter>
+        </service>
 
         <activity
             android:name=".player.BackgroundPlayerActivity"