MediaRouter2: Create MediaRouter2A.java for rewriting MediaRouter2

This CL introduces a draft class MediaRouter2A which will eventually
replace MediaRouter2.

Here are the reasons for introducing this class,
rather than directly editing MediaRouter2:
 - Introducing session/controller concept changes the role of
   MediaRouter2 a lot (e.g. no select/deselect, different callbacks),
   which makes rewriting much better than modifying current
   implementation.
 - Not to break current tests related to MediaRouterManager/Provider
 - Easier readability (no mixed code with previous implementation)

Bug: 146471105
Test: atest mediaroutertest:MediaRouter2ATest
Change-Id: I9dfe34fdb9d4530275b65d2805b6bdd0abcb662d