libc++fs is part of libc++ now. am: 160fbc4a97

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3116324

Change-Id: I11c702ccdd4c112ef22282e7e6c982e05157f332
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>