DisplayCutout: Ensure stable insets cover top cutout

Fixes an issue where the stable insets could become smaller
than the top cutout if the display density was set lower
than the natural density.

Bug: 78110564
Test: Set display size to "small", turn on cutout, verify apps do not have a gap between status bar and content.
Change-Id: I478047ccaade0e4e265cc57d03b072ae578b49d0