APIs to obtain SharedPreferences paths.

Needed for apps that want to migrate SharedPreferences from CE to DE
storage.  Note that a device will only ever enter a CE mode with a
factory reset, so apps should only be using these APIs when they
want to migrate files to a consistent location on non-FBE devices
for simplicity.

Bug: 25503089
Change-Id: Ic846215da1617d116a048e036415ac7ad523b770