identity: Fix occasional warning spew

It's possible to unschedule a timer when the timer is already dispatched,
but not yet holding the lock. If this happens, a precondition check
for the timer type may fail, leading to warning on the console.

This commit moves that precondition check to within the lock, and
also adds an explicit check to see if the timer has been unscheduled,
shortly after dispatch.

https://bugzilla.gnome.org/show_bug.cgi?id=709103