[Output Switcher] Fix list order for preference

Add system route to the top even Preference been set.
add check for isScanning to prevent exception

Bug: 286593457
Bug: 260055330
Test: make -j64 RunSettingsLibRoboTests ROBOTEST_FILTER="com.android.settingslib.media.InfoMediaManagerTest"
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:7c9c5f29f79ad3f4adcf21d6b46eb5cf43ae6449)
Merged-In: Ic7245f9c4169eabf63aa8eabac9b6a57fbc4ea7b
Change-Id: Ic7245f9c4169eabf63aa8eabac9b6a57fbc4ea7b