Exposes feature flag to wallpaper picker.

Wallpaper picker, through the shared library, can now query the feature
flag to know whether the customizable quick affordance feature is
enabled or not.

Bug: 254857639
Test: manually verified that turning the flag on and off on the system
UI side is reflected to WPPG with a visible or gone view and that
selecting affordances from WPPG still works

Change-Id: I2ea2baa4dbe935f1173648d673a64267ba8b0de2