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/linear_alloc.cc b/runtime/linear_alloc.cc
index e9db9b8..3f01fc3 100644
--- a/runtime/linear_alloc.cc
+++ b/runtime/linear_alloc.cc
@@ -16,7 +16,7 @@
 
 #include "linear_alloc.h"
 
-#include "thread-inl.h"
+#include "thread-current-inl.h"
 
 namespace art {