Product Bundle Identifier is not set in pbxproj

Expected: When adding ios platform (`cordova platform add ios`), the widget ID / bundle identifier should be copied to the Product Bundle Identifier value in the Build Settings.

Actual: Product Bundle Identifier is missing from project.pbxproj.

Steps to reproduce:

1. Create new Cordova project.
2. cordova platform add ios
3. Open up the generated xcworkspace in Xcode 9.3
4. Check the Target > Build Settings > Product Bundle Identifier and see that it's blank.
5. In Target > General, set Bundle Identifier to any value (or change it)
6. The Product Bundle Identifier gets set based on it

This caused me problems when using cordova-plugin-braintree, which sets a property in Info.plist using `${PRODUCT_BUNDLE_IDENTIFIER}`. In normal iOS development this would be the recommended way to parameterize a plist value based on the bundle ID.

May be tangentially related to CB-10186.