Refactor Trusty neverallow rules for clarity

- Separate rule sets that affect different things
- Clarify reason text
- Avoid duplication so future diffs touch less
- Correct plurality of func names

This CL is intended to make no functional changes.

Bug: None
Test: m
Change-Id: Ic68eb2657899e7106d252f8eafc8ced22b4e9baf