Pass "ravenwood-runtime" path from tradefed

- Stop discovery at runtime.
- I'm planning to use it in native code.
I don't want to use os.setenv() in Java because it's not thread safe.

Bug: 292141694
Flag: EXEMPT host test change only
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s
Change-Id: I8966f9bcf48ede82d56d1f9fdc039693dfbbe31d