Trebuchet : Decouple iconMask and iconBack dependency

Allow them to function independently in the case where
an icon pack only has one of them.

Change-Id: I7b72bd689eac7e411f88cb5fbacea3604d45036b
Merge "Trebuchet : Decouple iconMask and iconBack dependency" into cm-11.0
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_packages_apps_Trebuchet into cd-4.4