Inline MR2Manager into ProxyMediaRouter2Impl

This is a non-functional change and does not modify MediaRouter2Manager.
After this CL, ProxyMediaRouter2Impl will not depend on MR2Manager
anymore.

Test: atest CtsMediaBetterTogetherTestCases
Bug: 192657812
Change-Id: I41039f538fa28fbf25cf85948f3d38a920d6af4c