Start adding build rules for ravenwood

- Add "framework-minus-apex.ravenwood".
- Update the behavior of: --default-throw:
  - Now this will "keep" the subject classes and fields.
- When a 'throw' directive is specified on a static initializer,
  it's now handled as 'keep', because otherwise the class
  wouldn't be usable.

Test: ./scripts/run-all-tests.sh
Test: m framework-minus-apex.ravenwood
Bug: 292141694
Change-Id: If4d650dc175a5a78cf8844dfed2217c19f2d5790