SystemUI: Fix position and orientation of arrow keys in navbar

The arrow keys in the navigation bar during typing must keep their
position and orientation in inverted layout and/or RTL environment.
In the layout the left (right) button must stay 'left' ('right') not
be 'start' ('end'). Likewise the drawings should not be auto-mirror,
so special left/right drawings are added to the existing start/end
drawings which keep the correct orientation.

Change-Id: I0eed42d33ff431d7a6ca44555010481af9f03493