Themes: uninstallThemeForAllApps shouldn't check if map is empty if it's null

Potential NPE introduced in ec2200878706735cdd2caf4d934d988296998e85

Change-Id: I6d1ec165957c2be63086a18af2236e46978e9c8a
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