audio effect HAL: add getDescriptors method

Add an EffectsFactoryHalInterface  method getDescriptors()
taking an effect type as input and returning the list of decriptors
for all effects of the matching type.

Bug: 188502620
Test: atest AudioEffectTest

Change-Id: If817d08a787be149e46028e55d9873323cbc9d4b
3 files changed