gn_utils: cont. export proto import_dirs for all targets

Minor fix for aosp/3065103

Change-Id: I578eeaee1ad876179391e124ebf5e3d7941488e4

tools/gen_android_bp

diff --git a/tools/gen_android_bp b/tools/gen_android_bp
index d71b0dc..c8a2b1a 100755
--- a/tools/gen_android_bp
+++ b/tools/gen_android_bp
@@ -768,7 +768,6 @@
   # The .proto filegroup will be added to `tool_files` of rdeps so that the
   # genrules can be sandboxed.
 
-  tool_files = set()
   for proto_dep in target.proto_deps().union(target.transitive_proto_deps()):
     tool_files.add(":" + label_to_module_name(proto_dep.name))