Refactors LauncherPreviewRenderer to better support subclassing.

Basically this removes the inner class MainThreadRenderer, which
was only created and used for its populate() method in the parent's
getRenderedView() method. All methods and members of that subclass
are merged into the parent LauncherPreviewRenderer class, and
getRenderedView() simply inlines the previous populate() code.

Other smaller changes:
 - Extracted out shouldShowQsb() and shouldShowRealLauncherPreview()
 - Disables search view and its children to prevent interaction

Change-Id: I7d0cce73efbe022c16661a0ad66eefe5cb285641