Don't use String8::empty

With the current definition, it's an alias of default constructor. This
has very different meaning from std::string empty, especially that
static method String8::empty can still be called from instance context.

One of such bugs are present in in ConversionHelperAidl.

Bug: 295394788
Test: make checkbuild
Change-Id: Ic466986bdf850a44c30c919450fd8b92ac68758d
diff --git a/media/libaudioclient/AudioTrack.cpp b/media/libaudioclient/AudioTrack.cpp
index 0fa814c..20b47b4 100644
--- a/media/libaudioclient/AudioTrack.cpp
+++ b/media/libaudioclient/AudioTrack.cpp
@@ -3448,7 +3448,7 @@
     if (output != AUDIO_IO_HANDLE_NONE) {
         return AudioSystem::getParameters(output, keys);
     } else {
-        return String8::empty();
+        return String8();
     }
 }