MediaSession2: Change controller's behavior when connected

This stops calling
MediaController2.ControllerCallback.onPlaybackStateChanged() when the
controller is connected.

This is the preliminary step towards sending session's current values
to a controller when it's connected.

Bug: 72547163
Test: Run all tests once
Change-Id: I73b45268dba4ac1fe3fce99a575748db15f19168