HostStubGen: Stub generation is now optional, etc

- Now both --out-stub-jar and --out-impl-jar are optional
- Disable --enable-non-stub-method-check by default
  (it's not fully implemented anyway)
- Delete the script `run-ravenwood-test` since now atest
  just works.

Test: run-all-tests.sh
Bug: 292141694
Change-Id: I3b7d63600139425e5fffc12930ee860edf2acd7f