docs: New action views/action providers doc

Replaces the existing action views/action providers section in the
(soon to be removed) "ActionBar API Guide" page. Builds on
http://ag/730414 (which covers the basic app bar topics).

Some of the action providers doc is being moved into the
Javadoc reference for appropriate classes--that's being tracked
with CL http://ag/748902

See comment on patch set 2 for doc stage location.

Change-Id: Ia62b5155d7f67dc8922fc11917a12a957bf27fb1