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
diff --git a/media/libaudiohal/impl/EffectsFactoryHalHidl.h b/media/libaudiohal/impl/EffectsFactoryHalHidl.h
index 5fa85e7..ff26d9f 100644
--- a/media/libaudiohal/impl/EffectsFactoryHalHidl.h
+++ b/media/libaudiohal/impl/EffectsFactoryHalHidl.h
@@ -45,6 +45,9 @@
     virtual status_t getDescriptor(const effect_uuid_t *pEffectUuid,
             effect_descriptor_t *pDescriptor);
 
+    virtual status_t getDescriptors(const effect_uuid_t *pEffectType,
+                                    std::vector<effect_descriptor_t> *descriptors);
+
     // Creates an effect engine of the specified type.
     // To release the effect engine, it is necessary to release references
     // to the returned effect object.