The dark theme preference should have it's own screen rather than
being a dialog. This adds some boilerplate code that will be
needed for the illustration as well as converting the current
list preference to open a new screen.
Test: robotests
Bug: 128686189
Change-Id: I5b62276353c0d39ad2ad00d21d2280e76cceb09b
Having consistent import order will reduce chance of merge
conflict between internal and external master
Test: rebuild
Change-Id: I0b1a170967ddcce7f388603fd521f6ed1eeba30b
