Added support for multiple navigation key bars

System UI was build with the assumption that there is only one
navigation bar at a time, and that this bar is managed by
Phone UI, Tablet UI or TV UI.

This commit adds simple call back facilities at BaseStatusBar
for both, navigation key hints and menu key visibility change
events. These call backs are wired up in Phone UI, Tablet UI
and the TV UI stub.

This allows us to have more than on navigation key bar
in the UI and define those navigation bars directly
in BaseStatusBar, where it is available for all UIs.