Upgrade and cleanup

@oalami Please review and merge

This is the big upgrade PR for this sample. Included:
- Upgraded to Firebase 2.0.3
- Pulled the stuff the developer has to change out into one file
- Moved android context setting into an Application implementation, like our other samples
- Did a little bit of Android code style cleanup
- Expanded the readme, added a screenshot

I tested all the providers on a KK phone.

I'll follow up with one more PR that's just code formatting.