Standardise safety comments for unsafe blocks, and add some more. am: 9419ea4a34 am: d3fdcda72f am: cb3f154521 am: cdac6953fb am: 8f2519c4f6

Original change: https://android-review.googlesource.com/c/platform/system/librustutils/+/2652040

Change-Id: Ib9330c992ba70bfdb5563b6430159187651fb764
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>