Revert AlphaThresholdFilter to its old behavior wrt premul

TBR=borenet@google.com

This fixes the "serialize-8888 gm  imagealphathreshold_surface" failure related to:
https://codereview.chromium.org/1879643003/ (Switch AlphaThresholdFilter over to new onFilterImage interface)

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1885633003

Review URL: https://codereview.chromium.org/1885633003
1 file changed