Fix some force_dark prop behaviors

* Make it persist.hwui.force_dark so the dev option
  survices reboots.

* Remove the internal super-override behavior it applied
  using only the intended ViewRootImpl theme-aware behavior.

Fixes: 127350499
Fixes: 130354206
Test: set dev option, verified nothing went dark. Set dark
theme, verified non-dark apps were force-darked. Rebooted
and verified property still set.

Change-Id: I43adcbc25320669510fb03b162c98cbd14612f55