Add MediaRouterManager.Callback#onSessionReleased

Add the callback to let MediaRouter2Manager get notified
of released sessions.
A test is also added to check if the manager is aware of
a released session when the session is released from the
media router.

Bug: 157872573
Test: atest mediaroutertest
Change-Id: Ifb96b23ed61cc15f5015c453727a487c3b09a92b