Fix "null or empty system routes" crash in SysUI

This can happen when the media strategy is not available during startup,
so a legacy device route controller is instantiated, instead of the
"correct" audio manager route controller. This change fixes an
implementaiton issue in the legacy route controller, causing it to
return an empty collection of available routes, which is in turn used to
populate the provider info. This change makes the available routes
include the currently selected route.

Longer term, we want to get rid of the legacy route controller but, in
the meantime, this bugfix prevents a crash in sys ui.

Bug: b/357468728
Test: Manually by "forcing" a null media strat.
Test: atest LegacyDeviceRouteControllerTest
Flag: com.android.media.flags.enable_fix_for_empty_system_routes_crash
Change-Id: I5f84bbc9f1102b3df8966be27c7754cd05b53b6b