aaudio: process callbacks asynchronously

Spawn a thread to handle the disconnects for onRoutingChanged()
and onTearDown(). This is to prevent deadlocks when calling back into
AudioFlinger.

Bug: 155819038
Test: See bug for repro steps with OboeTester
Test: Also run TEST DISCONNECT in OboeTester. It was failing.
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: I83212fea140d3b6beca9aa737606a77a7c97b007