Cache Display.Mode so we can ignore refresh rate changes

DisplayCutoutView calls #update() when `onDisplayChanged` is called, but
if that were to be called _only_ because the refresh rate changes, then
we redraw the displaycutoutview (with no meaningful changes) and this
causes the display to change refresh rate again, starting the loop over.

Fixes: 155155363
Test: manual
Change-Id: Ic885cf24a9f4cc45970a3388653789abb3f592bb