SystemUI: Add EdgeGesture service.

Based on code of PieService.

Change-Id: I5c66e2858c8ddbe9faa0e0640c50714c8d1d5e29

Conflicts:
	core/res/res/values/pa_symbols.xml
SystemUI: Use EdgeGesture service to trigger Pie

EdgeGesture service triggers Pie when detecting gestures on
the WHOLE edge, not just the center.

Patch Set 2: Better trigger values

Change-Id: Id7d9d7f6f5a37ca3696cdddf0da78c88c067970e
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