Hygiene: Separate overlay code from mobile.stable.common

It should be possible to make overlays in desktop mode without loading
lots of unnecessary code. I noticed this
whilst trying to write a BetaFeature which uses overlays.

Change-Id: I436bca4d732c2dc2a2fb7a2ef057b268d0fac20f
Merge "Hygiene: Separate overlay code from mobile.stable.common"