Change activity to use alpha 0

.. instead of shrinking the bounds of the main window. This tests the
same functionallity, while being compatible with config_allowFloatingWindowsFillScreen

Bug: 338533031
Test: atest CtsWindowManagerDeviceActivity:ActivityRecordInputSinkTests
Change-Id: I3b2715fd439aab6c2f9e1962084f6ec9e3d83861