ART: Introduce thread-current-inl.h

Factor out Thread::Current() code into its own -inl file to remove
transitive includes.

This requires at the same time correcting mutex.h, i.e., moving
some functions into mutex-inl.h.

Test: m test-art-host
Change-Id: I88f888b604e0897368d9b483edce6ce4332dd9c9
diff --git a/runtime/mirror/reference-inl.h b/runtime/mirror/reference-inl.h
index a449b41..f8de6e6 100644
--- a/runtime/mirror/reference-inl.h
+++ b/runtime/mirror/reference-inl.h
@@ -20,6 +20,7 @@
 #include "reference.h"
 
 #include "obj_ptr-inl.h"
+#include "runtime.h"
 
 namespace art {
 namespace mirror {