Reduce namespace pollution.

This leaves us with just the mspace stuff and three libdex functions to clean
up. We deliberately expose the JII API, and I don't think there's anything we
can really do about the art_..._from_code stuff (and at least that starts with
"art_").

Change-Id: I77e58e8330cd2afeb496642302dfe3311e68091a
diff --git a/src/stringprintf.cc b/src/stringprintf.cc
index 8643c57..2a11437 100644
--- a/src/stringprintf.cc
+++ b/src/stringprintf.cc
@@ -4,6 +4,8 @@
 
 #include <stdio.h>
 
+namespace art {
+
 void StringAppendV(std::string* dst, const char* format, va_list ap) {
   // First try with a small fixed size buffer
   char space[1024];
@@ -61,3 +63,5 @@
   StringAppendV(dst, format, ap);
   va_end(ap);
 }
+
+}  // namespace art