Prevent unneeded scanning in SystemUI routing components

This change allows SettingsLib clients to receive routing state updates
without the need to actively scan, which should be only used by
components like the Output Switcher dialog.

As a follow up change, we should inspect Settings usages to remove any
remaining unjustified scanning.

Flag: ACONFIG remove_unnecessary_route_scanning DEVELOPMENT
Bug: 332515672
Test: atest com.android.settingslib.media
Change-Id: I9ad7aa0b52e4f36ca14f0f7ced35a33c0a7c5cad