Make MediaRouter2Manager provide correct system routes info

System route info could be changed per different apps. For example,
when a BT headset is connected, if a media app call
MediaRouter.selectRoute(defaultRoute) via Jetpack lib, the app will use
the phone speaker while the other media apps use the BT headset.

System UI should be changed depends on this CL.

Bug: 156549746
Test: atest mediaroutertest
      atest CtsMediaTestCases:android.media.cts.SystemMediaRouter2Test
Ignore-AOSP-First: AOSP doesn't have SystemMediaRouter2Test
Change-Id: Ie24f97023b7113a2b59f0b55ab7b4230b394e9bd