gn2bp: stop parsing dep tree for builtin modules

Targets that are part of builtin_deps will be removed / replaced by
gen_android_bp. So if such a target is encountered while parsing the gn
desc output, all of its dependencies can be safely ignored.

Test: builds
Change-Id: I5bd954c2b95e2ae53a4ac9dc420a067fd76da9a7
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index c57524d..5f3cf85 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -1478,7 +1478,7 @@
     log.basicConfig(format='%(levelname)s:%(funcName)s:%(message)s', level=log.DEBUG)
 
   targets = args.targets or default_targets
-  gn = gn_utils.GnParser()
+  gn = gn_utils.GnParser(builtin_deps)
   for desc_file in args.desc:
     with open(desc_file) as f:
       desc = json.load(f)