Fix task bar crash in gesture nav mode with task bar / nav bar unification on

a11yButton is reserved for 3-button mode so it can be null. Also TaskbarNavLayoutter is used for large screen both in gesture nav and 3-button mode, so need to check isContextualButtonShowing to add contextual buttons

Fixes: 300849078
Test: Make sure that it doesn't crash in large screen gesture nav mode
Change-Id: I9d0694b7f4c95461f24efb2af1af905e68e3cd8d