Fix String16 operator+ to actually work

This change fixes the String16 utility's operator+ function.

Change-Id: Ibed7b07277b50bf178febc72b6ce497893e66f17
diff --git a/include/utils/String16.h b/include/utils/String16.h
index 584f53f..360f407 100644
--- a/include/utils/String16.h
+++ b/include/utils/String16.h
@@ -156,7 +156,7 @@
 
 inline String16 String16::operator+(const String16& other) const
 {
-    String16 tmp;
+    String16 tmp(*this);
     tmp += other;
     return tmp;
 }