Media: Add MSM#getMediaKeyEventSession

Add MediaSessionManager#getMediaKeyEventSession
for system apps to get the current media key event session,
which may be different from the first session in
MediaSessionManager#getActiveSessions().

Bug: 181383356
Test: atest MediaSessionManagerTest
Change-Id: I56ece75e352150693a5658e2660f39b5702d8612