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

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

Change-Id: Ie02b5f72c7c4b037e6e5b2f3699abeb3333a37d7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>