Fix depth jumping around during transitions.

This is caused because we use mDepth for depth comparisons, but there are
cases where we set mDepth but we do not pass that value to WallpaperManager
(ie. surface is null or not valid) and that leads us into
inconsistent states.

Bug: 155780358
Change-Id: I3faf14416d5783ad472892425eb0bd37dd469a46