Upgrade pffft to b9819ffacac25e62727fc854f8c78522fb4eb2b3 am: 56310291a5 am: f1dc982446 am: d53d0fafda

Original change: https://android-review.googlesource.com/c/platform/external/pffft/+/1536831

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I15eab281869bb3db3965d9bae71483e7bd391be1