Enable dark mode feature flag

Enabling the flag, since there isn't a reasonable fallback.
If setup wizard passes ManagedProvisioning a dark mode theme, and
the feature flag is off, then the setup wizard screen templates
would be in dark mode, but MP's resources would still
be in light mode.

Bug: 157458872
Test: manual
Change-Id: I5c5fee93a17082987751a4d7473fd68161c9d9cb