Rebuilds ActionBar accessibility description as needed

Previously, ActionBarView only updated the contentDescription for the navigate
up/home affordance when it is enabled or disabled.  This causes that description
to be rebuilt and applied whenever the title or subtitle of the view is changed.
This also inadvertantly removes whitespace in ActionBarView.

Bug: 8757286
Change-Id: I1ea02cad20e0d5c0c57d071727cc647c4dd52d44