Rework permissions to be retained when an app is temporarily uninstalled.

This allows us to keep the assigned permissions when apps are temporarily
removed due to the SD card being unmounted, and also if you use the
facility to uninstall an app but keep its data.

Also fixes issue #2515189: Potential permission spoofing attack in
Android (external bug 7166)

Change-Id: I2a120ec938552028c989f9e0e890c32773957738