Fix multi-display nav bar tests on large screen devices

Tests failed because #createnavigationBar skip create nav bar even on
the secondary display if there's a task bar initialized.
This CL changes to only check on the default display instead.

fixes: 241509471
Test: atest MultiDisplaySystemDecorationTests#testNavBarShowingOnDisplayWithDecor
Test: atest MultiDisplaySystemDecorationTests#testNavBarNotShowingOnDisplayWithoutDecor
Test: atest MultiDisplaySystemDecorationTests#testNavBarNotShowingOnPrivateDisplay
Change-Id: Ibdd5fb65a92915ac92e908c0091260573cec18c8