Fix create colors in Wu quantizer

This line is suspiciously complicated compared to other 3 ints -> int
representing color functions. This patch ensures the colors produced
have an explicitly opaque alpha channel (transparent pixels are filtered
out earlier in the quantization process), and ensures we're not
unnecessarily masking the red/green/blue channels.

Test: Add log dump to output hex for all colors created, immediately
after the Wu quantization finishes, thus isolating it from the larger
quantization process. Test a variety of wallpapers, make sure hex codes
make sense, verify that the fix holds by converting
Bug: 182333325

Change-Id: Ic0bd4afcd095845e00eb6d10d05aab336266d6c3