Enhancements to Shadow Service.setForeground() notification handling

Some useful tweaks I added to help me with testing foregrounding of my service. In particular, the <code>Notification<.code> that you supply in <code>setForeground()</code> is actually registered with the <code>NotificationManager</code> - making behaviour more closely match that of real Android. Plus added one-arg convenience version of <code>getNotification()</code> to <code>ShadowNotificationManager</code> to parallel the one-arg version of <code>notify()</code>.