MediaRouter2: Add (un)registerSessionCallback

This CL pulls out the process of registering SessionCallback
from the method MediaRouter2#requestCreateSession.

Now the registered callback can be reused until unregistered.

Also, this renames existing (un)registerCallback methods to
(un)register'Route'Callback.

Bug: 146400872
Test: atest mediaroutertest
Change-Id: I68f71b0a857780ab9e2a0ca2a550af5ab4962f70