Handle @RavenwoodSupported to show APIs supported by...

subclasses on the dashboard.

- This is implemented as "throw" but with "supported" in the stats label.

- Also updated the comment on HSG test annotations.

- Also added "comment" field for @RavenwoodKeep, etc, just for documentation.

- Also deleted the RELEASE_TARGET_JAVA_21 golden files. (only support 21 now)

- This should support class-wide too, which will be a follow-up.

Bug: 414821464
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s
Flag: TEST_ONLY
Change-Id: I0dc7f9648ecb2d45905c9f3142cb4a003ebd9f37