[Output Switcher] Fix InfoMediaManager route listing preference updates

Also, upon starting scanning, do update the map even if the
route listing preference is null, so that the map is cleared.

Test: make -j64 RunSettingsLibRoboTests ROBOTEST_FILTER="com.android.settingslib.media.InfoMediaManagerTest"
Bug: 285597089
Change-Id: Ic0e7b306ef5ff8a87f90b9fb8a7af9a0e82f1fa0