Fix full backup/restore detection of encrypted devices

The confirmation UI did not request the needed permission, so was failing
to communicate with the mount service; as a "safe" failure mode, it was
assuming the device was encrypted.  Fixed; now it presents the correct
prompt text for the device's encryption state.

Bug 5958195

Change-Id: Ic03db16673b89d3377e0362a09cf51bfb572d78b