Flag desktop stashing behavior

Turn off desktop stashing by default. When going home from desktop, user
exits the desktop and subsequent app launches will be to fullscreen.
Keep the stashing behavior behind a flag so that it can be enabled if
desired.

Bug: 292109910
Test: atest DesktopTasksControllerTest
Change-Id: I566c33c4787542da70bcc052d3c3f2788ea90c4b