lockscreen: Allow to swipe back to incoming calls

I accidentally swiped away an incoming call and couldn't swipe back on the lockscreen (had to unlock to accept the call). The reason is that we were looking at PHOSH_CALL_STATE_ALERTING rather than PHOSH_CALL_STATE_INCOMING. Let's require a recent enough calls so we don't need to care about both values.

/cc @devrtz does that look right?