Improvements from !934

Some more getters around layer-surface (which might be useful for other users like squeekboard or phosh-osk-stub also decrufting PhoshLockscreen.

This is useful even without !934 (merged) in yet and makes reabsing !934 (merged) even simpler.

Signed-off-by: Guido Günther guido.gunther@puri.sm