Upgrade rust/crates/unicode-segmentation to 1.7.1 am: 56cb1af3b1 am: ef88bfccf1

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/unicode-segmentation/+/1547936

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I4af1ab0a30c8faa21a9651506db772cf80bc4295