Decouple zooms and blurs on app launch

We'd like to be able to disable blurs during app launch, but without
disabling zooms as well. The previous sysprop would set the depth of
BackgroundAppState to 0, when what we want is to conserve wallpaper
zoom.

Bug: 191969790
Test: adb shell setprop ro.launcher.blur.appLaunch false
Change-Id: Ie4b26096f6ac723c3981bba2829557e6cc6c733b