Move the binding out of GerritGlobalModule so it's part of the daemons,
making it easier to swap to a different PermissionBackend.
Expose DefaultPermissionBackend class as public so another backend
could wrap and delegate to DefaultPermissionBackend by accepting it
as an argument to its constructor.
Change-Id: I01f10d8a65d8942e73354bcf055c26b17d2606f9
0c80f2a6c3