Remove callback upon MediaSession#release

This change prevents all callbacks from MediaSession after
the app calls MediaSession#release. Concretely, this change
fixes flakiness in MediaControllerTest, where the callback
implementation tries to access the media session after its
corresponding field has been set to null.

Bug: 294178159
Test: atest CtsMediaBetterTogetherTestCases
Change-Id: I112fa5df6aafdb2774ed65c2d08ab1eb680e6b47

media/java/android/media/session/MediaSession.java

@@ -442,6 +442,7 @@ public final class MediaSession {
      * but it must be released if your activity or service is being destroyed.
      */
     public void release() {
+        setCallback(null);
         try {
             mBinder.destroySession();
         } catch (RemoteException e) {