AudioFlinger: Use atomic pointer for AudioFlinger singleton.

Set when AudioFlinger is assigned to sp<> (i.e. onFirstRef())
by the main audioserver thread.

Test: basic audio works
Test: AudioEffectTest AudioPreProcessingTest BassBoostTest
Test: EnvReverbTest EqualizerTest LoudnessEnhancerTest
Test: PresetReverbTest VirtualizerTest VisualizerTest
Bug: 161341295
Change-Id: I21b8c88f35fbeaae6214b2f894cfcea22b965f6f