Keep wallpaper quick switch order stable (2/3).

When switching wallpapers via the quick switcher, keep the order of the
options from changing. What's happening is that there is a silent
recreation of the activity (one that cannot be caught via
onConfigurationChanged) which is causing everything to be reloaded.

The CL changes the view-model to be a true Android ViewModel that can be
retained across activity instances.

Bug: 262924643
Test: manually verified that the wallpaper option remain stable
during/after wallpapers are switched and that the reset system still
works.

Change-Id: I8e63eb9e9534130a2b88f8749cf93d0c5057d149