Enable the MPRIS server plugin - allows control of android media players

The changes allow loading the plugin on older Android versions

src/org/kde/kdeconnect/Plugins/MprisReceiverPlugin/MprisReceiverPlugin.java

@@ -34,6 +34,7 @@ import org.kde.kdeconnect.Helpers.AppsHelper;
 import org.kde.kdeconnect.NetworkPacket;
 import org.kde.kdeconnect.Plugins.NotificationsPlugin.NotificationReceiver;
 import org.kde.kdeconnect.Plugins.Plugin;
+import org.kde.kdeconnect.Plugins.PluginFactory;
 import org.kde.kdeconnect.UserInterface.AlertDialogFragment;
 import org.kde.kdeconnect.UserInterface.StartActivityAlertDialogFragment;
 import org.kde.kdeconnect_tp.R;
@@ -44,17 +45,16 @@ import java.util.List;
 import androidx.annotation.Nullable;
 import androidx.annotation.RequiresApi;
 
-@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
-//FIXME: Breaks on Android 4 because it extends OnActiveSessionsChangedListener
-//@PluginFactory.LoadablePlugin
-public class MprisReceiverPlugin extends Plugin implements MediaSessionManager.OnActiveSessionsChangedListener {
-
+@PluginFactory.LoadablePlugin
+@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP_MR1)
+public class MprisReceiverPlugin extends Plugin {
     private final static String PACKET_TYPE_MPRIS = "kdeconnect.mpris";
     private final static String PACKET_TYPE_MPRIS_REQUEST = "kdeconnect.mpris.request";
 
     private static final String TAG = "MprisReceiver";
 
     private HashMap<String, MprisReceiverPlayer> players;
+    private MediaSessionChangeListener mediaSessionChangeListener;
 
     @Override
     public boolean onCreate() {
@@ -68,7 +68,9 @@ public class MprisReceiverPlugin extends Plugin implements MediaSessionManager.O
             if (null == manager)
                 return false;
 
-            manager.addOnActiveSessionsChangedListener(MprisReceiverPlugin.this, new ComponentName(context, NotificationReceiver.class), new Handler(Looper.getMainLooper()));
+            assert(mediaSessionChangeListener == null);
+            mediaSessionChangeListener = new MediaSessionChangeListener();
+            manager.addOnActiveSessionsChangedListener(mediaSessionChangeListener, new ComponentName(context, NotificationReceiver.class), new Handler(Looper.getMainLooper()));
 
             createPlayers(manager.getActiveSessions(new ComponentName(context, NotificationReceiver.class)));
             sendPlayerList();
@@ -83,8 +85,9 @@ public class MprisReceiverPlugin extends Plugin implements MediaSessionManager.O
     public void onDestroy() {
         super.onDestroy();
         MediaSessionManager manager = (MediaSessionManager) context.getSystemService(Context.MEDIA_SESSION_SERVICE);
-        if (manager != null) {
-            manager.removeOnActiveSessionsChangedListener(MprisReceiverPlugin.this);
+        if (manager != null && mediaSessionChangeListener != null) {
+            manager.removeOnActiveSessionsChangedListener(mediaSessionChangeListener);
+            mediaSessionChangeListener = null;
         }
     }
 
@@ -169,18 +172,20 @@ public class MprisReceiverPlugin extends Plugin implements MediaSessionManager.O
         return new String[]{PACKET_TYPE_MPRIS};
     }
 
-    @Override
-    public void onActiveSessionsChanged(@Nullable List<MediaController> controllers) {
+    private final class MediaSessionChangeListener implements MediaSessionManager.OnActiveSessionsChangedListener {
+        @Override
+        public void onActiveSessionsChanged(@Nullable List<MediaController> controllers) {
 
-        if (null == controllers) {
-            return;
-        }
+            if (null == controllers) {
+                return;
+            }
 
-        players.clear();
+            players.clear();
 
-        createPlayers(controllers);
-        sendPlayerList();
+            createPlayers(controllers);
+            sendPlayerList();
 
+        }
     }
 
     private void createPlayer(MediaController controller) {