Ignore low chroma colors when calculating a hue's population

We rank/score colors for suitability for theming choices, and the top
4 are presented to the user.

The score has two components: the chroma, or colorfulness, of the color,
and the hue population of the color — what percentage of the image has
hues within 15 degrees of the color.

A wallpaper that is mostly black, green leaves, and a light dash of low
chroma red on a couple leaves, ends up ranking red as the highest choice
because black has a red hue.

Ignoring low chroma colors when calculating the hue population avoids
this issue — the dash of low chroma reds no longer benefits from their
hue being close to black, the majority of the image.

Bug: 201980078
Test: Push wallpaper from bug comment #4 to device, check color options.
Red is no longer offered as an option. To come: apply to Monet Studio,
verify no ill effects

Change-Id: Ib40a6646077e07c2cc85da339186d9a307709def