Android: implemented PlatformPrintJobController.onComplete

[Doflatango]

Connection with issue(s)

Resolve issue:
Webview can only be disposed when printing job finished, this PR given a chance for listening when printing job get done on Android.

printJobController.printCurrentPage(settings: PrintJobSettings(handledByClient: true));
final completer = Completer();
printJobController.onComplete = (completed, error) async {
    printJobController.dispose();
    completer.complete();
};
await completer.future;
// Webview can only be disposed when printing job finished
headlessInAppWebviewController.dispose();

To Do

• double check the original issue to confirm it is fully satisfied
• add testing notes and screenshots in PR description to help guide reviewers
• request the "UX" team perform a design review (if/when applicable)