Create template interface and separate local/proxy routers

This is a non-functional change. This change builds on ag/22381003 by
separating special method implementations into `LocalMediaRouter2Impl` and
`ProxyMediaRouter2Impl`, which implement a common `MediaRouter2Impl`
interface. This change starts separating the logic between regular and
privileged routers.

Bug: 192657812
Test: atest CtsMediaBetterTogetherTestCases
Change-Id: Idcced629460a075bbf4a5d700678e1458ea63a57