Ignore transparent pixels during WSMeans

The WSMeans quantizer uses starting clusters from a Wu quantizer as
starting points for an optimized Kmeans algorithm.

The first thing the quantizer does is assign pixels from the image being
quantized to those starting clusters. Either due to the image source or
downscaling, transparent pixels may be in the pixels. Filter those out:
they create misleading results, such as there being two colors in the
image, semi-transparent black and green, in a wallpaper that is solid
green by all accounts.

Fixes: 182333325
Test: atest CtsAppTestCases:android.app.cts.WallpaperColorsTest#fromDrawableTest
passes locally on sunfish. link: http://ab/I26300007868608112

Change-Id: Icaebd98eeba27d3dc59a698282dd3be03fce1bf9