Bluetooth: Mesh: use settings priority feature

By using `SETTINGS_STATIC_HANDLER_DEFINE_WITH_CPRIO` we can ensure that
BT subsystems always get loaded after BT, and BT Mesh after both of
them.

This solves the host having to register a GATT service in a delayed
manner, as we are sure to now register the GATT service after sc_commit
sets `SC_LOAD`.

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