Upgrade rust/crates/rand to 0.8.0 am: 8f9af988c5 am: 838c6d95c7

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/rand/+/1534331

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I4b9ab893e1f8ca938ee98d2e75ea6ea4f78a71b2