Refactor oat.h.

Move OatClassType and OatMethodOffsets to oat_file.h and
make all dependencies on oat.h explicit instead of relying
on transitive #includes. This reduces the number of files
that need to be rebuilt for oat version changes.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I89c8e397a4f423043d7a036a995cc33548b25051
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 691aa80..908a928 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -146,6 +146,7 @@
 #include "native_bridge_art_interface.h"
 #include "native_stack_dump.h"
 #include "nativehelper/scoped_local_ref.h"
+#include "oat.h"
 #include "oat_file.h"
 #include "oat_file_manager.h"
 #include "object_callbacks.h"