Revert change to force_dark property

Settings can't write to persist.* without special
selinux rules. Instead for debug simplicity just
switch back to debug.hwui.force_dark and let it
reset on reboot.

Fixes: 131697927
Test: toggle override-force dark in dev options
Change-Id: Ieac6edb2a7b444fc2f63d5d4f1b657bad6ead409