Adjusted the systembar display in mock game test app to only hiding statusbar...

Adjusted the systembar display in mock game test app to only hiding statusbar while under immersive mode.

Sometimes the systembar will be automatically hid in a short time after
swiping down to show the systembar under immersive mode. That will cause
the quick switch gesture failed since the navigation bar is hidden. So,
adjusted the system bar hiding behavior to only hiding Statusbar to avoid
flakiness.

Test: https://android-build.corp.google.com/abtd/run/L95200030006266256
Bug: 342654085
Flag: TEST_ONLY

Change-Id: I22609318c47513d40a67d71dd8906cb6323557cd