ART: Clean up thread.h and thread_list.h

Remove dependency on stack.h and gc_root.h. Remove unused object
callbacks include. Factor out ManagedStack into its own set of files.
Fix up users of transitive includes.

Test: m test-art-host
Change-Id: I01286c43d8c7710948c161b1348faabb05922e59
diff --git a/compiler/optimizing/code_generator.h b/compiler/optimizing/code_generator.h
index c2b2ebf..14f5865 100644
--- a/compiler/optimizing/code_generator.h
+++ b/compiler/optimizing/code_generator.h
@@ -31,6 +31,7 @@
 #include "nodes.h"
 #include "optimizing_compiler_stats.h"
 #include "read_barrier_option.h"
+#include "stack.h"
 #include "stack_map_stream.h"
 #include "string_reference.h"
 #include "type_reference.h"