MediaRouterService binds services when necessary

MediaRouterService maintained bindings to provider services
once those are established.

With this CL, it only binds services when there is a non-empty
discovery preference set by a foreground app or an app is casting.

This change may break output switcher, which enables transfer media
of background apps.

To alleviate that MediaRouter2Manager#startScan and #stopScan are added
so that system UI can force the service bind to the services and find
remote devices to cast.

Bug: 169575701
Bug: 172920557

Test: manually and CTS
Change-Id: I4a47fdb1c9fe05a04d26950485833c9cbfb91a69

media/java/android/media/IMediaRouterService.aidl

@@ -73,6 +73,8 @@ interface IMediaRouterService {
     void unregisterManager(IMediaRouter2Manager manager);
     void setRouteVolumeWithManager(IMediaRouter2Manager manager, int requestId,
             in MediaRoute2Info route, int volume);
+    void startScan(IMediaRouter2Manager manager);
+    void stopScan(IMediaRouter2Manager manager);
 
     void requestCreateSessionWithManager(IMediaRouter2Manager manager, int requestId,
             in RoutingSessionInfo oldSession, in @nullable MediaRoute2Info route);

media/java/android/media/MediaRouter2Manager.java

@@ -146,6 +146,36 @@ public final class MediaRouter2Manager {
         }
     }
 
+    /**
+     * Starts scanning remote routes.
+     * @see #stopScan(String)
+     */
+    public void startScan() {
+        Client client = getOrCreateClient();
+        if (client != null) {
+            try {
+                mMediaRouterService.startScan(client);
+            } catch (RemoteException ex) {
+                Log.e(TAG, "Unable to get sessions. Service probably died.", ex);
+            }
+        }
+    }
+
+    /**
+     * Stops scanning remote routes to reduce resource consumption.
+     * @see #startScan(String)
+     */
+    public void stopScan() {
+        Client client = getOrCreateClient();
+        if (client != null) {
+            try {
+                mMediaRouterService.stopScan(client);
+            } catch (RemoteException ex) {
+                Log.e(TAG, "Unable to get sessions. Service probably died.", ex);
+            }
+        }
+    }
+
     /**
      * Gets a {@link android.media.session.MediaController} associated with the
      * given routing session.

media/java/android/media/RouteDiscoveryPreference.java

@@ -153,6 +153,7 @@ public final class RouteDiscoveryPreference implements Parcelable {
             return false;
         }
         RouteDiscoveryPreference other = (RouteDiscoveryPreference) o;
+        //TODO: Make this order-free
         return Objects.equals(mPreferredFeatures, other.mPreferredFeatures)
                 && mShouldPerformActiveScan == other.mShouldPerformActiveScan;
     }