MediaRouter: Defer releasing routing controller when transfer

In order to allow media router to communicate with the old routing
controller for synchronizing media status during transfer,
this CL postpone releasing the old routing controller.

Basically media router should release the old routing controller by
itself when it's done with the old routing controller.
If it didn't, the old routing controller is automatically released
after timeout (30s for now)

This CL also clarifies "old routing controller" in onTransfer callback.
Now it is consistent regardless of who requested transfer.

A request from MediaRouter2Manager is handled by MediaRouter2 to
simplify session creation logic.
Limitation of this approach is that a routing session unknown to
MediaRouter2 can't be transferred to a different provider but it is out
of R scope so it's okay.

Bug: 158713035
Test: CTS test && atest mediaroutertest && manually transfer using
support v7 demos

Change-Id: I168af69e2a25240227aea0fd9e892eaa91e78ee3