Bind to system media MR2RouteProviders for any manager scans

This changes makes the system bind to any system media route providers,
even if no route discovery preference are set. That's because system
media routing doesn't require apps to declare support for routing.
System media routing is transparent to the app.

Note that this change is non-obviously flagged. The field that indicates
whether a provider supports system media routing is flagged before it's
passed to the MR2ProviderService constructor.

Bug: b/362507305
Test: atest CtsMediaBetterTogetherTestCases CtsMediaHostTestCasts
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I329fae281ea421a81f5902ceebd8e55ecde0252a