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.
Merge pull request #125 from alfonsosanchezbeato/fix-off-on-crash

voicecall: Take notice of SIM removal