Use APP_ID in xcconfig rather than PRODUCT_BUNDLE_IDENTIFIER

This is required because when we add an app extension to the
project, there will be two PRODUCT_BUNDLE_IDENTIFIER fields
in our project file, so we cannot use xcconfig to set that
key directly.

Signed-off-by: Roopesh Chander <roop@roopc.net>

WireGuard/WireGuard/Config/Developer.xcconfig.template

@@ -3,5 +3,7 @@
 // You Apple developer account's Team ID
 DEVELOPMENT_TEAM = <team_id>
 
-// The bundle identifier of this app
-PRODUCT_BUNDLE_IDENTIFIER = <app_id>
+// The bundle identifier of this app.
+// Should be an app id created at developer.apple.com
+// with Network Extensions capabilty.
+APP_ID = <app_id>