Animation timer can report wrong time to animations

Specifically, if the last running animation is unregistered (startStopAnimationTimer.start(STARTSTOP_TIMER_DELAY)), a lot of work happens, and then a new animations is registered before the timer has fired, the new animation may never play (because the elapsed time is reported as very high).

This was causing failures in the tst_qdeclarativebehaviors autotest on the CI system.