[rilmodem] When sim stays on in offline mode PIN cache can be emptied

After modem is configured to keep SIM alive in offline mode PIN
cache can and should be tied to SIM presence notification. Note:
inserting this fix before modem configuration change will
effectively break PIN caching part of the hidden reset.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>