aaudio: stop calling virtual methods from destructor

Was calling close(), which is abstract and virtual.
This may have been related to some audioserver crashes.
Also cleaned up some strong pointer handling.

Bug: 63390734
Bug: 63353455
Test: run CTS nativemedia/aaudio many times
Change-Id: Ib95aed60a64771b64395c67f0921c67146f9d10f