ahat: getImmediateDominator should not return the SuperRoot.

We should not be exposing the SuperRoot instance anywhere through the
public API, because the SuperRoot is not part of the public API. Change
getImmediateDominator to return null for "rooted" instances instead of
the SuperRoot instance to better match the documented behavior.

Test: atest ahat-tests, with new test added.
Change-Id: Id93a09bd714d7b00c6c50233f01d1af7b4258fae
2 files changed