Place handwriting surface below gesture monitors

Until WM specifies an order for display overlays, we manually place the
handwriting surface below gesture monitors so that handwriting cannot
block events from being sent to gesture monitors.

This CL also does some other minor cleanup.

Bug: 217538817
Test: None
Change-Id: I65a66ce53d1c3227f502fa7910037ddd6fc7b58a
1 file changed