libaudiohal: Implement simple HIDL client

This client requires both HIDL client and server to be of the same
ABI version (e.g. 32-bit).

EffectHalHidl::process and EffectHalHidl::processReverse are stubs.

Some extra warnings are logged due to unimplemented functions
in the legacy HAL.

Bug: 30222631
Change-Id: Id074131dec6d6c4f1031695abcf3ea066e92a5af
Test: Play Music (play / EQ), YouTube, Loopback RTLT, Ok Google on N5X
diff --git a/include/media/AudioParameter.h b/include/media/AudioParameter.h
index 9719efa..1ace607 100644
--- a/include/media/AudioParameter.h
+++ b/include/media/AudioParameter.h
@@ -88,6 +88,7 @@
     status_t get(const String8& key, String8& value) const;
     status_t getInt(const String8& key, int& value) const;
     status_t getFloat(const String8& key, float& value) const;
+    status_t getAt(size_t index, String8& key) const;
     status_t getAt(size_t index, String8& key, String8& value) const;
 
     size_t size() const { return mParameters.size(); }