MediaRouter2: Enhance readability of MediaRouter2ServiceImpl

This CL introduces following changes:
 - Renamed methods by adding 'Router2' or 'Manager',
   in order to easily distinguish the type of the caller.
 - Rename and reordered MediaRouter2ServiceImpl methods
 - Add missing @NonNull/@Nullable

Bug: 149642195
Test: CTS and atest mediaroutertest
Change-Id: I82758c1bb54e70b699e06fa46fa3a97462e41bc1