Sorry you can't do it in iOS, however in Android, you can, all you need to do is write platform specific code. 
You could create an Activity with the Theme.Dialog theme. In your AndroidManifest.xml file add the theme to the activity, like this:
<activity android:name=".DialogActivity" android:theme="@android:style/Theme.Dialog"></activity>
From your service simply start this Activity. You will have to start the activity with the Intent.FLAG_ACTIVITY_NEW_TASK flag. See How to start an Activity from a Service
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