Remove settings in overview mode for Launcher3.

The appereance of the settings button in overview mode is now dependent on
a overridable method, such that we can have it in GEL, but not in Launcher3.
This change also adjusts the logic of how wide the overview panel is, depending
on how many items we have.

Bug: 12441429
Change-Id: Ibf537c26602785f565292cbfa213eb123469ebcd