Fine tune splash screen icon generation algorithm

Filter out the translucent pixels when compute the foreground colors
of an adaptive icon, which should make sense because the translucent
pixels on icon should only used for decoration, do not let it affect
the opaque pixels, also with this filter we won't drop down the
performance when using palette.

Bug: 193304878
Test: manual launch single color background apps.
Change-Id: Ia9873cf907d30bb7f30318a289a54159d0a51ee3