OverlayCategoryPreferenceController: Change the way we set overlays

Before this patch, we were just setting overlays on the fly
without saving them anywhere persistent, which ended up
removing font and icon shape customizations on wallpaper change
or device reboot.

Modify Settings.Secure.THEME_CUSTOMIZATION_OVERLAY_PACKAGES
instead and SystemUI will apply the overlays automatically.

Test: Change wallpaper, observe font and icon shape customizations
      are still applied.
Change-Id: Idc50dc1cea1cdb239a0949425f14277c6d1ce332