Added check to state machine that if modem is attached to a network
it is considered to be registered to a network and state machine
can continue to next states. This fixed issues seen in IoT network
that network does not allow registering if already attached.
CellularDevice class own new state machine. Now we don't have to expose state machine
which may change and so we don't have to make API changes if that happens.
EasyCellularConnection uses now CellularDevice instead of old state machine.