Update dark theme to have new screen

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
10 files changed