Make STL demo overlays closer to the Flexi dual shade

This CL makes the overlays of the STL demo closer to the actual
Flexiglass dual shade:
 - they are both aligned at the end of the screen.
 - the QS overlay is below the Notification overlay.
 - the QS overlay is shown when swiping down from the top end edge.
 - the notification overlay is shown when swiping down outside of the
   top end edge.

This CL forked the SplitEdgeDetector.kt used by Flexi.

Bug: 353679003
Test: Manual in the demo app
Flag: EXEMPT demo app
Change-Id: I81f43baa32c3022ad0f1556cfbf193dbdae48091