Improvements on phone finder action

[germede]

This pull request builds upon #40 and addresses:

1. ForegroundService not starting automatically (at least on my phone)
2. Notification instead of AlertDialog because otherwise the phone keeps ringing in loop and cannot be stopped (when app's activity is not active but it's just listening through the foreground service).
3. PhoneFinder as a standalone utils class / object, like Flashlight or CameraCapture.

[germede]

Link #42