Build `ravenwood-junit` against `test_current`.

Some CTS tests desire to link against `test_current` to prove to
themselves that they're not touching hidden APIs.  This means if they
also want to use our `ravenwood-junit` rules, that dependency also
needs to link against `test_current`.

Since we're reaching down into plenty of hidden details when we're
actually running on a Ravenwood runtime, we split into a `stub` and
an `impl` library, so that we give a cleanly compiling library to
to clients, which is then replaced under the Ravenwood runtime.

Bug: 292141694
Test: atest-dev CtsTextTestCasesRavenwood CtsTextTestCases
Change-Id: I96dbc9643324a7853a992e97b51a88f70eb721f3