Block setting wallpapers from managed profiles.

Silently fail when a managed profile app tries to change the
wallpaper and return default values for getters in that case.
This is implemented through a new AppOp that is controlled by
a new user restriction that will be set during provisioning.

Bug: 18725052
Change-Id: I1601852617e738be86560f054daf3435dd9f5a9f