Letterbox: Ignore light status bar flags for letterboxed windows

Fixes an issue where letterboxed windows would still apply the
light status bar flag, even though the status bar was letterboxed
to black.

Bug: 65689439
Test: Enable cutout emulation, open "all apps" on launcher, verify status bar icons are visible
Change-Id: I9ce9560eedcc9cc2c1907478f7c4f4a1002c7235