MediaRouter2: Implement RouteSessionController#release

This CL implements followings:
 - RouteSessionController#release
 - Router side logic of calling SessionCallback#onSessionReleased()

The test should be added when MediaRouterService actually notifies
the clients of session release. (i.e. No new test added.)

Bug: 146400872
Test: atest mediaroutertest
Change-Id: I2bad73e00986903b8c925704b0144d9e75f3bbf7