Bluetooth: GATT: Ensure GATT service is registered first

Accourding to Bluetooth specification the Service Changed
Characteristic shall not have its handle changed once it has been
bonded, so this moves the GATT service to be the very first service
registered that way it is guaranteed that it won't change even if
device is flashed with a different configuration which end up changing
the handles after it.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>