Deprecate ActionBar navigation modes

Navigation modes were a great shortcut for patterns that are no longer
recommended or that don't integrate as nicely with application
content. Apps have done a great job both inventing new navigation
patterns that work well in their own contexts or following patterns
that can be implemented inline with an app's content. Navigation modes
as a prescriptive construct are no longer needed.

Be free, apps!

Change-Id: I9ae1c2efc830c7dcb6664061bba68b1748928ae3