Bluetooth: Mesh: Proxy: Fine-tune subnet advertising rotation

Create a slightly smarter algorithm for choosing how long to advertise
each subnet. This is particularly important for the mesh_shell app,
since it uses a 10 second NODE_ID_TIMEOUT, meaning starting Node ID
advertising through user interaction would only succeed in advertising
one subnet (due to this being configured to 10 seconds).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>