MediaSession2: Fix random NPE in MediaBrowser2Test

MediaSessionService#getSession() may be null until the
MediaSessionService#onCreate(), and its session callback can be called
before that. (e.g. System service can try connecting to the session
in another thread)

Don't use MediaSessionService#getSession() when it's unclear whether
the MediaSessionService#onCreate() is finished.

Bug: 74177171
Test: Run MediaComponents test 10 times
Change-Id: I89f3d1a59f84f5d950a14a7b412cfbbdad2c8bee

media/java/android/media/MediaSessionService2.java

@@ -173,10 +173,12 @@ public abstract class MediaSessionService2 extends Service {
     /**
      * Get instance of the {@link MediaSession2} that you've previously created with the
      * {@link #onCreateSession} for this service.
+     * <p>
+     * This may be {@code null} before the {@link #onCreate()} is finished.
      *
      * @return created session
      */
-    public final MediaSession2 getSession() {
+    public final @Nullable MediaSession2 getSession() {
         return mProvider.getSession_impl();
     }