Use hidl_string conversion constructor as test.

Test: hidl_test
Change-Id: I626c40e1a3121e4f41ba9f612efc7321f7c03f85
diff --git a/tests/foo/1.0/default/Foo.cpp b/tests/foo/1.0/default/Foo.cpp
index 235a12d..f855f21 100644
--- a/tests/foo/1.0/default/Foo.cpp
+++ b/tests/foo/1.0/default/Foo.cpp
@@ -68,10 +68,7 @@
         doStuffAndReturnAString_cb _cb) {
     ALOGI("SERVER(Foo) doStuffAndReturnAString");
 
-    hidl_string s;
-    s = "Hello, world";
-
-    _cb(s);
+    _cb("Hello, world");
 
     return Void();
 }