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

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

Change-Id: I81151189960104ee40c64daa9565d1a714362c47
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>