More conservative dark text calculation

Refactored WallpaperColors to use constrast ratio instead of luminance
for detecting dark pixels. Also using a contrast more conservative than
what GAR requires while decreasing the dark area threshold.

Change-Id: I67b799be4b7ccd50bb3e63c6179d513b9b76446b
Fixes: 76435920
Test: manually set various wallpapers
Test: use new debug flag to verify which pixel is actually dark