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/scoped_thread_state_change-inl.h b/runtime/scoped_thread_state_change-inl.h
index ed6e349..aa96871 100644
--- a/runtime/scoped_thread_state_change-inl.h
+++ b/runtime/scoped_thread_state_change-inl.h
@@ -22,6 +22,7 @@
 #include "base/casts.h"
 #include "jni_env_ext-inl.h"
 #include "obj_ptr-inl.h"
+#include "runtime.h"
 #include "thread-inl.h"
 
 namespace art {