Revamp MediaRouter2 scanning API and add support for screen off scanning

This change adds support for companion-app-related routing use cases and
addresses limitations with the previous scanning mechanisms. The design
and motivation is described in go/revamp-mr2-scanning-api.

Test: atest CtsMediaBetterTogetherTestCases
Bug: 281072508
Change-Id: I3045c7213a29f7f35b578391cfe19b49aa9748dc
Flag: ACONFIG com.android.media.flags.enable_screen_off_scanning DEVELOPMENT