Improving pie control's user interaction

User feedback showed that it is sometimes difficult to interact
with pie controls. This commit adds some tweaks to how pie
controls react on user touch events.

This changes:
* Pie trigger on LEFT and RIGHT are now more forgiving in the
  initial phase of a swipe. (This should reduce problems when
  trying to activate the pie with a thumb.)
* Pie items are now more sloppy when it comes to focus.

Change-Id: Ib21556ed7f7ff9c6cdbcd1760e94a17c8534706a