Pie controls.

Based on the multiple navigation key bar patch, this commit adds
basic pie controls. The implementation is inspired
by Paranoid Android but is completely rewritten.
The aim was to get as little intersection with existing code as
possible and provide a farily easy interface to extent these
controls by new slices (inherited from PieSlice).

The pie controls are only active when expanded screen is turned on.

Based on this commit there is another one that adds more optimizations
for Phone UI to pie controls. And there is a seperate commit for
the settings app, too.
Add pie control optimizations for Phone UI.

This commit brings some pie control tweaks to the Phone UI. These
tweaks make the pie controls react to several conditions that are
are special to the Phone UI. That means that this piece of code
is more deeply embedded within the SystemUI code.

This mostly adds the foloowing behaviour, that is Phone UI
dependent:
* Disable BOTTOM trigger when notification area is expanded
* Disable LEFT and RIGHT trigger when a keyguard is active
* Enable search light feature