Change AacAdtsExtractor inheritance

AacAdtsExtractor derived from MediaExtractor but didn't actually
integrate with the extractor service in any way, and so didn't
really need to use or implement the MediaExtractor API. Make it
an independent class so it will be unaffected by any future
MediaExtractor API changes.

Bug: 67908544
Test: build, boot
Change-Id: I612a4f6e722a6ef55e542bf80f36f312b944580d