State machine has retry logic until device is attached to network.
After this CellularContext does the context activation e.g. connect.
There was no retry logic for context activation. Added logic to
CellularContext level so it's available for at and (upcoming)ril layers.
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.