Add support for privileged routing across users

After this change, users of privileged routing with
INTERACT_ACROSS_USERS_FULL will be able to control the routing of other apps
across users.

Also, the new API getInstance(Context, Looper, String, int) allows callers to
specify their own Looper.

Test: atest CtsMediaBetterTogetherTestCases
ProxyMediaRouter2HostSideTest
Bug: 288580225
Bug: 192657812

Change-Id: I055914a3d5cea3dbd0268b82c983f14ab87d559d