aaudio: pass smart pointer to mixer thread

Before this, the mixer thread could close the stream using a stale pointer.
This could cause timeouts when plugging headphones in or out.

Bug: 76092458
Test: write_sine_callback -m3 -pl -s1000
Test: unplug and replug headphones many times on Marlin
Change-Id: I437eb756686446a75fc5d04bc09d9408816585de