Fix nav bar tests on device without soft nav bar

The original implementation is to wait for default nav bar showing and get current
nav bar states. However, it fails on device with physical nav bar.
Change the condition to wait for launcher instead.

Test: atest MultiDisplaySystemDecorationTests
fixes: 138156492
Change-Id: Ibf0117718de6edcc9a7301f001dcc70926fb4b8f
1 file changed