Generate flx files in the gen directory. (#3595)

This allows tools to locate them more easily.

build/flutter_app.gni

@@ -79,7 +79,7 @@ template("flutter_app") {
   flutter_tools_bin = "$flutter_tools_out_dir/dart-tools/flutter_tools"
 
   dot_packages = "$target_gen_dir/$dart_package_name.packages"
-  bundle_path = "$root_out_dir/$bundle_name"
+  bundle_path = "$target_gen_dir/$bundle_name"
   bundle_depfile = "$target_gen_dir/$bundle_name.d"
   snapshot_path = "$target_gen_dir/${target_name}_snapshot.bin"
   snapshot_depfile = "${snapshot_path}.d"
@@ -223,7 +223,7 @@ template("flutter_aot_app") {
   flutter_tools_bin = "$flutter_tools_out_dir/dart-tools/flutter_tools"
 
   dot_packages = "$target_gen_dir/$dart_package_name.packages"
-  bundle_path = "$root_out_dir/$bundle_name"
+  bundle_path = "$target_gen_dir/$bundle_name"
   bundle_depfile = "$target_gen_dir/$bundle_name.d"
   assembly_path = "$target_gen_dir/$target_name.S"
   assembly_depfile = "${assembly_path}.d"