Pie controls: Tweaking UX for pie service (1/2)

With the introduction of pie service there is the need to
fine tune the parameters for activation again.

Patch Set #2:
* Base trigger thickness set to 10dp
* User settable activation sensitivity

Patch Set #4:
* Reverted #setSensitivity() in PieGestureTracker
* Added Sensitivity "Very High"

Change-Id: Ief7c1786467195f2ad52dcdb0efd3ad03a48834c