Clear preferred feature when unregistering MediaRouter2

MediaRouter2Managers' preferred feature didn't get cleared even when the
MediaRouter2 is unregistered/died. As a result, when an app is
reinstalled/relaunched, the routes for the previous process are still
shown in the MediaRouter2Manager.

This CL fixes the problem.

Bug: 158824569
Test: Passed CTS and mediaroutertest
      Also tested with reinstalling support7Demos.
Change-Id: Ib5d84c41862177af32931e0d69b02f461105bba6

media/java/android/media/MediaRouter2Manager.java

@@ -576,6 +576,10 @@ public final class MediaRouter2Manager {
     }
 
     void updatePreferredFeatures(String packageName, List<String> preferredFeatures) {
+        if (preferredFeatures == null) {
+            mPreferredFeaturesMap.remove(packageName);
+            return;
+        }
         List<String> prevFeatures = mPreferredFeaturesMap.put(packageName, preferredFeatures);
         if ((prevFeatures == null && preferredFeatures.size() == 0)
                 || Objects.equals(preferredFeatures, prevFeatures)) {