Align taskbar to the left/right in 3 button nav for certain devices.

Otherwise we position the taskbar in the center.

Bug: 259712417
Fixes: 267997547
Test: transient taskbar unaffected
      persistent taskbar
      - test on small tablet
      - test on large tablet
      - test LTR and RTL

Change-Id: Ieb0a304d9963ebf583bc4ef2deaab747019e8d6d