audio: Fix StreamOut ownership in default wrapper

StreamOut::asyncCallback could became an owner of StreamOut
causing the destructor to be called on the offload callback
thread, while the legacy HAL is holding a mutex, which resulted
in a deadlock.

Removed erroneous usage of sp<StreamOut> in asyncCallback.
The legacy HAL joins the offload callback thread when closing
output stream, thus StreamOut destructor is guaranteed to finish
only after the offload callback thread has exited, and using
a raw pointer to StreamOut inside asyncCallback is correct.

Bug: 70863217
Change-Id: I0d77018cf3df5ad07251732733288d425dd836eb
Test: manual