Standardise safety comments for unsafe blocks, and add some more. am: 9419ea4a34 am: d3fdcda72f am: cb3f154521 am: cdac6953fb am: 8f2519c4f6 am: 486456fff4

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

Change-Id: I5cf9d4a22c40b390fb1f6175500703f8e170b489
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>