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/libmediaplayerservice/include/MediaPlayerInterface.h b/media/libmediaplayerservice/include/MediaPlayerInterface.h
index 1cbd8a0..994c64a 100644
--- a/media/libmediaplayerservice/include/MediaPlayerInterface.h
+++ b/media/libmediaplayerservice/include/MediaPlayerInterface.h
@@ -147,7 +147,7 @@
         virtual bool        needsTrailingPadding() { return true; }
 
         virtual status_t    setParameters(const String8& /* keyValuePairs */) { return NO_ERROR; }
-        virtual String8     getParameters(const String8& /* keys */) { return String8::empty(); }
+        virtual String8     getParameters(const String8& /* keys */) { return String8(); }
 
         virtual media::VolumeShaper::Status applyVolumeShaper(
                                     const sp<media::VolumeShaper::Configuration>& configuration,