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/dex2oat/dex2oat_test.cc b/dex2oat/dex2oat_test.cc
index 6420aa8..40ca875 100644
--- a/dex2oat/dex2oat_test.cc
+++ b/dex2oat/dex2oat_test.cc
@@ -28,6 +28,7 @@
 
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/mutex-inl.h"
 #include "dex_file-inl.h"
 #include "dex2oat_environment_test.h"
 #include "dex2oat_return_codes.h"