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: I86ded320d4adc88a610749d23e116c833e207b17