Bluetooth: Mesh: enable scanner before enabling proxy gatt

Once proxy gatt is enabled, due to the nature of gatt advertisements
being continuous, the controller will reject starting the scanner after
enabling proxy gatt. As such we should start the scanner first.

Currently the order works as we are using k_work to postpone the proxy
gatt service registration, because services can't be registered between
initializing BT and loading the settings.
Once we start using the new settings priority feature, we are sure to
envoke enabling proxy gatt after loading the settings, so the delayed
enabling can and should be removed.

This preventetive fix has been tested by running relevant bsim tests
after rearranging in which order the settings of the subsystems are
loaded via `h_commit`.

Signed-off-by: Kyra Lengfeld <kyra.lengfeld@nordicsemi.no>