Revert "In the CPU backend, stop treating alpha images as coverage" am: fa93c4addd am: cb6873e5f1 am: 6d7cfb270d am: 0e33d0a640

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/skia/+/19294092

Change-Id: I0785fc06ba8b90ec5b217e2126c14bd02b3f2047
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>