Set dark theme flag when initializing with Color

Users want to be able to use this flag and have been relying on
WallpaperColors#fromBitmap given that flags were not parsed by
the main constructor.

Fixes: 151717291
Test: atest WallpaperColorsTest
Change-Id: I0470a867c54756df9a758b24835a50bc67191050
2 files changed