Force inlining on trivial accessors.

Make volatility for GetFieldObject a template parameter.
Move some trivial mirror::String routines to a -inl.h.

Bug: 14285442

Change-Id: Ie23b11d4f18cb15a62c3bbb42837a8aaf6b68f92
diff --git a/runtime/reference_table.cc b/runtime/reference_table.cc
index a3119bb..11527fa 100644
--- a/runtime/reference_table.cc
+++ b/runtime/reference_table.cc
@@ -23,7 +23,7 @@
 #include "mirror/class.h"
 #include "mirror/class-inl.h"
 #include "mirror/object-inl.h"
-#include "mirror/string.h"
+#include "mirror/string-inl.h"
 #include "thread.h"
 #include "utils.h"