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.h b/src/stringprintf.h
index 413fef5..19b006b 100644
--- a/src/stringprintf.h
+++ b/src/stringprintf.h
@@ -6,6 +6,8 @@
 #include <stdarg.h>
 #include <string>
 
+namespace art {
+
 // Returns a string corresponding to printf-like formatting of the arguments.
 std::string StringPrintf(const char* fmt, ...)
         __attribute__((__format__(__printf__, 1, 2)));
@@ -17,4 +19,6 @@
 // Appends a printf-like formatting of the arguments to 'dst'.
 void StringAppendV(std::string* dst, const char* format, va_list ap);
 
+}  // namespace art
+
 #endif  // STRINGPRINTF_H_