Add MediaRouter2#stop

MediaRouter2#stop stops the current media routing and
invokes the callback onStopped.
According to the change transferTo doesn't accept null route
as its argument.

CTS is changed as well.

Bug: 150472061
Test: CTS tests related to media router2

Change-Id: I6bc7b7ad394189e035e4c018677efb94e5497c85