Update transparent region when cutouts update

The transparent region is used to compute the blocking region for the
DPU. This needs to be updated when the cutouts change.

Call updateProtectionBoundingPath in onAttachedToWindow. It is called
the other times we call updateCutout.

Fixes: 225212638
Test: debug.screenshot_rounded_corners
Change-Id: Ib8e7e303b257947074ca9714f2cdaf3a3b7880ab