Remove new TestAPI in NotificationRankingUpdate

Only a single testAPI function was added; no other part of this class is
in the public API surface.

For simplicity, as such, I think it's probably best that I just remove
the function from the TestAPI.

Bug: 304478750
Test: Annotation only change; atest NotificationRankingUpdate
to ensure we're not breaking unit tests

Change-Id: I1e3860adf44f25bdfe43efe7f3ad432c9f1b04e6