docs: Removed ActionBar API guide

Removed the now-obsolete ActionBar API guide; it's been
superseded by the AppBar training class
(http://developer.android.com/training/appbar/ )

Changed the most important ActionBar links and references to point to
the AppBar docs (or some other appropriate page).

A separate CL will set up redirects from the ActionBar API guide.

See first comment for doc stage location.

bug: 25194804
Change-Id: I0859ea1712971fcf2f6cf131b2a5221b5bf3ea44