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/libdatasource/MediaHTTP.cpp b/media/libdatasource/MediaHTTP.cpp
index 58c1ce8..b1f2140 100644
--- a/media/libdatasource/MediaHTTP.cpp
+++ b/media/libdatasource/MediaHTTP.cpp
@@ -156,7 +156,7 @@
 
 String8 MediaHTTP::getUri() {
     if (mInitCheck != OK) {
-        return String8::empty();
+        return String8();
     }
 
     String8 uri;