AAudio: bump priority of AAudio Mixer to match FastMixer

Service RT thread should run at a slightly higher priority because
it generally does less work and is more critical.

Bug: 71817389
Test: Run an AAudio stream in SHARED mode.
Test:   adb shell ps -e -T -o uid,pid,tid,ppid,pri,ni,cmd| grep AAudio
Test: See bug for more test instructions.
Change-Id: I762af6ac1a9883d47b4c42bfe9f8455416251a12