Unfinished play-services fix
Grabs the advertising Id on a background thread

- Play Services can't run on the main UI thread, so we get the
  advertising Id on a background thread and then add it to the Subject.
  If an event is sent right after a Subject is created, there is a high
  chance the advertising Id won't be added for that initial event.
  Once the Id is received, it will be added to the following events
  sent.
- This is not a complete fix for the Play Services branch. One more fix
  needs to go in to detect if the Play Services is made available.
- Ref: #16