Bluetooth: gatt: Reuse existing handles from a previously added service

When removing a service using bt_gatt_service_unregister, its attributes
handles will keep their value which can cause a problem when adding the
service again to the GATT database.
When re-adding, the gatt_register routine is taking the last handle of
the GATT database to compare it with the handles of the service to be
added.
If a service has handles with a lower value than the last handle of
the database an error will occur.
If we add/remove/add the last service, no error will occur as its
handles are always greater than the last one of the database.

Instead of resetting the handles when unregistering a service and
reassign new ones we make sure that the handles of the services are
not in use in the GATT database in order to place the service back
where it used to be.

Signed-off-by: Arthur Crepin-Leblond <arthur.crepin@proglove.de>