voicecall: Take notice of SIM removal

In case the SIM atom was destroyed and then re-created, the voicecall
atom crashed because it had an old sim_context structure. This fix adds
a callback that is called from the SIM atom so we know when it is
destroyed.
rilmodem: Enter PIN before unlocking it

We enter the SIM PIN in case it has not been entered yet when unlocking
the PIN. This homogenizes the unlock feature for different modems, as
in some cases the modem firmware already enters the PIN before
unlocking but in other cases it does not.
Merge pull request #127 from alfonsosanchezbeato/enter-pin-on-unlock

Enter PIN before unlocking it
Merge pull request #125 from alfonsosanchezbeato/fix-off-on-crash

voicecall: Take notice of SIM removal