Allow TabHosts to have no children in the tabcontent widget

Replace the code which throws an exception if there are no children in
the FrameLayout that is the tabcontent, with code to add a single
dummy tab.

This makes the TabHost renderable in those scenarios where the real
tabs are added dynamically rather than via XML.

Change-Id: I72442bd7d40eddd875b3507585c1f372fcae3329