EffectBase: Make callback getter/setter atomic

Plain sp<> not thread-safe for concurrent set and get.

Test: AudioEffectTest
Test: AudioPreProcessingTest
Test: BassBoostTest
Test: EnvReverbTest
Test: EqualizerTest
Test: LoudnessEnhancerTest
Test: PresetReverbTest
Test: VirtualizerTest
Test: VisualizerTest
Bug: 190024493
Change-Id: Ic65f8eabe21453aefefbdd0080e66d9cb7184f43