Revert "In the CPU backend, stop treating alpha images as coverage" am: fa93c4addd

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

Change-Id: Ic950ee194ff74320f87f1cf853017caf4043d9dc
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>