PreferenceFragment showing the breadcrumb when not needed.

The breadcrumb section is used for a single pane mode for showing
the current user location. The breadcrumb should not be shown if
its title and subtitle are empty. The code in PreferenceActivity
onCreate updates the breadcrumb with the title and subtitle in
a single page mode but does not hide it if they are empty. This
change fixes that.

bug:11242762

Change-Id: Ib1ba9e8afd9d0ab2e85d78a65ca7c18489464f6d