Allow in-progress AOT processing to be detected

It would be useful to be able to detect where AOT processing is being performed, not just that it has been performed. The latter is covered by `AotDetector` but we don't have anything for the former. As discussed with @snicoll and @jhoeller, `AbstractAotProcessor` could set a system property or the like which can then be used to detect that AOT processing is in progress.