notif: Use live value for app ID on registering APNs token

[AhmedTareek]

Fixes #407

Changes made:

• Replaced the hardcoded app bundle ID with packageInfo.packageName.
• Added a null check for packageInfo with an assert for debugging.

This change caused the "token initially unknown" test in store_test.dart to fail due to the added asynchronous call to ZulipBinding.instance.packageInfo, which delayed the completion of registerToken.

To resolve this, a Future.delayed(Duration.zero) was added after await startFuture in the test to ensure _registerNotificationToken completes before the test checks for the request.

await startFuture;
// This delay is needed to allow the listener callback to `store._registerNotificationToken()`
// that occurs as a side effect of `startFuture` setting the token value to complete.
await Future<void>.delayed(Duration.zero);
if (defaultTargetPlatform == TargetPlatform.android) {
  checkLastRequestFcm(token: '012abc');
} else {
  checkLastRequestApns(token: '012abc', appid: 'com.zulip.flutter');
}

This minimal delay ensures the asynchronous operations within the test complete in the expected order, resolving the test failure.

[gnprice]

Thanks for the contribution!

Before we can review this in detail, it will need tests. See the guidance in our README file about writing tests.

[AhmedTareek]

Tests are added.