Merge Navigation Bar and Pie Controls button config (2/2)

Based on the commit series for pie controls, this commit
makes the Navigation Bar and Pie Controls share the same
button layout.
This commit applies the changes made in part 1 to the
Settings app. For the real commit see framework/base.

This commit introduces here:
* Some setting constants changes to avoid clashes with PA.
* Removal of the pie search key setting.

Change-Id: I5b4abd6d32a3fc835466ac4cb1f0b9c851641061

Conflicts:
	res/xml/system_settings.xml