Disable task override density in desktop windowing

Changing task density on enter/exit desktop mode causes some app compat
issues with apps that can't handle the configuration change or don't
handle it at all.
Hides the feature behind an adb flag that's disabled by default.

Flag: EXEMPT bugfix
Bug: 344599474
Test: manual - change Display Size in settings and verify desktop tasks
and their window decor change sizes
Test: atest DesktopTasksControllerTest DesktopModeWindowDecorationTests

Change-Id: Ibef0c9c60b84c84f20bc1294fb0f16f09e6199cc