MediaRouter: clear session info when disconnected

When MediaRoute2ProviderProxy disconnects, it should
clear its session info but it didn't.

This CL corrects it such that sessions are released correctly.

In addition to that, it also releases the previous controllers when
transfer w/ MediaRouter2Manager.

Test: manually
Change-Id: If827a9473d79e3a154688c0dfaa3075b0e0b6ef5