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

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

Change-Id: I3903a52d39fe08e68874b6903986faa66c3fa075
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>