Bluetooth: Mesh: Fix binding between proxy related states

Fix binding between GATT Proxy, Node Identity and their private
counterpart states according to sections 4.2.45.1 and 4.2.46.1
accordingly. When non-private state is enabled, the private counterpart
is disabled. The reverse binding prohibits change of non-private state.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>