This is the last thing we want. :-)
1. Dialog boxes from Services
One of the best experiences in mobile devices, IMHO, and Android in particular, is that after decades, we got rid of system-wide, pesky alert dialogs. Finally, best practices [1, 2] for user interaction gave us a way to avoid the infamous disseminate use of MessageBox(hwnd, lpText, lpCaption, uType), competing for focus and for the attention of the poor user. See video parody above.
The reason it feels awkward to start a dialog from a Service is exactly because it is supposed to be a background task, without user interaction. By concept, you shouldn't be doing this. That's the reason why we see these tricks (transparent activities, what a silly thing) to cheat the design guidelines in the first place. They are bad, they disrupt the user experience, they steal focus and attention. They disrupt our work.
2. Use notifications instead
Whenever you want to notify a user of something from the background, when the user is somewhere else, you use a notification. It's the default pattern, and it doesn't bother the user.
Therefore, you should be sending notifications from your Service.
From there, if the user is interested, then he will touch the notification and you start your own activity, possibly resuming your activity, creating a new one, and then using a dialog requesting action to be performed, or whatever you want to do.
3. Finally, do NOT use FLAG_ACTIVITY_MULTIPLE_TASK
You should not, ever, use this flag unless you have carefully read and fully understood the documentation, and the implications of using that flag.
Do not use this flag unless you are implementing your own top-level application launcher.
(...)
Because the default system does not include graphical task management, you should not use this flag unless you provide some way for a user to return back to the tasks you have launched.
Really. In this case, just don't.
