...so that the lower-layer "dconf:*" reads from the .../system/ dconf tree while
the final-layer "dconf:!" reads from and writes to the .../user/ dconf tree.

Using a single tree would not really work:  For one, a sysadmin will want to
finalize a property so it cannot be changed by extensions; that means that
property must be set and finalized in the lower-layer "dconf:*".  But for
another, a user will want to change a property for which an extension provides a
value; that means that property must be set in the final-layer "dconf:!".  So
the two "dconf:*" and "dconf:!" must store their respective data in different
places.

Change-Id: I9029e7f779bcb86e8f26cfc22401e97e0cb3362b