Change TestActivity to not be immersive

Focus tests will fail if the immersive window is shown on top.
Dismissing the immersive window doesn't to seem to work in all cases,
likely due to a race.

Instead, no longer make the activity immersive.

Test: requestFocusWithMultipleWindows
Bug: 313674425
Change-Id: Idc9d3031a75f59fc1ce8591a168d56217f9c5b48