FlingTracker can divide by zero and hang StatusBar in an infinite loop

if 2 events have the same timestamp then FlingTracker in PanelView will divide
by zero causing NaNs. the bug is usually triggered by rapid StatusBar up/down
dragging activity and updateCarrierLabelVisibility can then loop indefinitely
looking at NaNs. the user visible symptom is that StatusBar refuses to pull
down and is burning cpu time.
Fix divisions by zero on boot.

Change I976ffae3a56d411b00026f86b16efa9743a23293 introduced a problem
where depending on the rotation settings the WM override display
configuration would be filled with invalid values.
Fix that by correctly checking that the display is ready prior to
setting the override configuration.