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/mutex.cc b/src/mutex.cc
index 666c368..d41c6cb 100644
--- a/src/mutex.cc
+++ b/src/mutex.cc
@@ -99,7 +99,7 @@
 }
 
 pid_t Mutex::GetTid() {
-  return art::GetTid();
+  return ::art::GetTid();
 }
 
 ConditionVariable::ConditionVariable(const std::string& name) : name_(name) {