Bluetooth: Mesh: Fix sending beacons to Friend Queue & GATT clients

Whenever there's a security change (Key Refresh or IV Update) we
should immediately send beacons to any Friend Queues or connected GATT
clients. Introduce a helper function to do this, and make sure it's
called from all places that change the Key Refresh or IV Update
states.

This fixes test case MESH/SR/PROX/PB-12-C.

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