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/thread_pool.cc b/runtime/thread_pool.cc
index e051e76..8349f33 100644
--- a/runtime/thread_pool.cc
+++ b/runtime/thread_pool.cc
@@ -30,7 +30,7 @@
 #include "base/stl_util.h"
 #include "base/time_utils.h"
 #include "runtime.h"
-#include "thread-inl.h"
+#include "thread-current-inl.h"
 
 namespace art {