* Move Brave Shields utility functions
- Move utility functions that are not using `//content/` from `/components/brave_shields/content/` to `/components/brave_shields/core/` so it can be also be accessed by iOS.
* Update tests
- Update tests after moving functions from `/components/brave_shields/content/` to `/components/brave_shields/core/`.
* Fix UNSAFE_TODO
- Update brave_shields_p3a.cc to address unsafe buffer warnings.