Add a test class validator to ravenizer

For now, just detect tests extending legacy base class, but just show
warnings.

Flag: EXEMPT host test change only
Bug: 292141694
Test: m $( frameworks/base/ravenwood/scripts/list-ravenwood-tests.sh )
    and manually check the output
Change-Id: I689b97129c655f5feee3ce789477f493e995b853