Make aidl annotate onTransact with @Override

BUG: 1902262
diff --git a/tools/aidl/AST.h b/tools/aidl/AST.h
index aec2164..3156356 100755
--- a/tools/aidl/AST.h
+++ b/tools/aidl/AST.h
@@ -22,6 +22,8 @@
     FINAL           = 0x00000020,
     ABSTRACT        = 0x00000040,
 
+    OVERRIDE        = 0x00000100,
+
     ALL_MODIFIERS   = 0xffffffff
 };