Ravenwood/HostStubGen refactor and bug fixe

- Bug fix: Now native substitution methods can handle bytes correctly.

- Spec change: disallow using DisabledOnNonRavenwood on types.

It turned out the device side test runner can't handle it if a class rule
uses Assume.

We could still support it with a non-class rule, but to avoid confusion,
let's disallow using it on classes for the time being.

- Add more tests

- Some refactoring for future changes.

Test: ./ravenwood/scripts/run-ravenwood-tests.sh
Bug: 292141694
Change-Id: I6e2fd389ef63d31472f5eafcd9a58ac9e52e8117