AudioEffects: Annotate smart pointer member variables

Indicate which smart pointers are const and which ones may have
issues with concurrent modification and access.

No material difference, but running tests to be sure.

Test: AudioEffectTest AudioPreProcessingTest BassBoostTest
Test: EnvReverbTest EqualizerTest LoudnessEnhancerTest
Test: PresetReverbTest VirtualizerTest VisualizerTest
Bug: 161341295
Change-Id: I6cac21893cd39496b2c2ab24fb203abe9e9e663e