Suppress immersive mode cling in LockTask mode.

We don't want to show the immersive mode cling when the device is in
LockTask mode, as the status bar swipe gesture is unavailable.

Bug: 68305547
Test: manual, with DEBUG_SHOW_EVERY_TIME
Change-Id: Id11516267800e0b0c8de1da52a43b2dded5181be