Clean up SecurityLevel module

When I have some spare time in the future, I should clean up the SecurityLevel.sys.mjs module to:

1. Make it easier to read.
2. Add proper js docs.
3. Have a clearer distinction between private and public methods.
4. Stop returning unused return values from bindPref.
5. Use observer signals to notify android and desktop consumers of a change in security level, instead of requiring consumers to listen to preference changes. Allowing the underlying preferences to be contained to this module, and make the consumers less dependent on internal details.

/cc @ma1 @pierov