overview: Use monotonic time to check for consecutive activations

overview: Use monotonic time to check for consecutive activations

We don't toggle the overview if the request happens too close to the
last activation, to filter out double-clicks or activation by both
the hot corner and a click. However as the check is based on the
real time, the check breaks if the system clock moves backwards and
the last activations appears to be in the future. Fix this by using
monotonic time which is guaranteed to only move forward.

https://bugzilla.gnome.org/show_bug.cgi?id=763886