ART: Add object-readbarrier-inl.h

Move some read-barrier code into a new header. This prunes the
include tree for the concurrent-copying collector. Clean up other
related includes.

Test: mmma art
Change-Id: I40ce4e74f2e5d4c692529ffb4df933230b6fd73e
diff --git a/runtime/gc/system_weak_test.cc b/runtime/gc/system_weak_test.cc
index 9b601c0..dfbbd2a 100644
--- a/runtime/gc/system_weak_test.cc
+++ b/runtime/gc/system_weak_test.cc
@@ -23,6 +23,7 @@
 #include "base/mutex.h"
 #include "collector_type.h"
 #include "common_runtime_test.h"
+#include "gc_root-inl.h"
 #include "handle_scope-inl.h"
 #include "heap.h"
 #include "mirror/string.h"