Don't cleanup overlay mapping for non-theme apps

mOverlay structure should keep all app/theme mappings even
if the app does not exist. Otherwise we will have to insert back the mapping
for every app that is installed which would be time consuming.

Without this patch an upgrade app loses its applied theme
until the device is rebooted.

Change-Id: Id02b8e0a3bdd78d1a9d39cd38294c584ef95f8db