audioflinger: Move async callback setting to onFirstRef

Since initializing async callback involves passing
PlaybackThread's 'this' around, it's safer to do that
in 'onFirstRef' instead of the constructor. Since
'onFirstRef' runs before the thread loop is started,
shared data access model is not affected.

Bug: 206737711
Test: atest CtsMediaAudioTestCases:AudioTrackOffloadTest
Test: check offloaded playback on the device
Change-Id: I26f76b9ff843c871911fe3546027cb197f1f0272