security.sharedsecret: Add rust derives and fix docs. am: 442a08d3ee am: 97b86f8dab am: c732eaff9f

Original change: https://android-review.googlesource.com/c/platform/hardware/interfaces/+/1647987

Change-Id: Id6def2c2dddf6bec5f1663e61cab546864ca4a5d