Guard app handle hiding on immersive changes behind a flag

Hiding & showing the app handle in response to immersive mode changes
causes flakiness in LayoutTests. This change puts the logic behind a
flag so that it's disabled until the test failures are fixed.

Fix: 335832294
Bug: 336368019
Test: atest
CtsWindowManagerDeviceAnimations:android.server.wm.animations.LayoutTests#testLayoutAfterRemovingFocus
--rerun-until-failure (passes with 200+ attempts with flag disabled)
Test: manual - with flag enabled, the app handle is shown/hidden on
immersive mode changes

Change-Id: Iebc6498e4726d3edeb33d246353a00cbd230ee72