SystemUI: Position Pie at gesture's triggered position

As the EdgeGesture service is triggered at the whole screen
edge, place Pie at the gesture's triggered position, instead
of always positioning it at the center of the edge.

This might be controversial and a matter of taste, but in
particular on tablets it's sometimes hard to hit the exact
center of the edge and therefore gestures often fail. Could
also imagine enabling this feature on tablets only. Needs
some discussion...

Change-Id: Ife28ac434667148583b34bcccd11262c111b5a67
Merge "SystemUI: Position Pie at gesture's triggered position" into kitkat