Fix syncing back to at mode after ppp disconnect.
Fix AT_CellularContext flags and states to allow new connect after disconnect.
Fix that state machine is not reseted in disconnect is it's running (might be
running because of another context or new connect already started).
Moved methods to classes CellularDevice and CellularInformation.
SIM interface was removed to simplify cellular usage and
methods better suite new classes.
Updated greentea and unit tests.