Utilize Dependency Injection for ContactsRepository

[SuhasDissa]

I know you didn't expect this, but I wanted to utilize some dependency injection.

• Moved the code from ContactHelper into a ContactRepository
• Created a singleton instance of each repository inside the Application class
• Created ContatsSource enum to switch between Device and Local Repositories

There's no clear performance benefit from doing this, but the overall code looks cleaner and easy to understand.
And since the repository is a singleton now, we can do some caching to improve performance in the future.

PS: I had to do all the changes in one commit because the code was too intermingled.

Whether or not you use the changes is up to you. I just wanted to give you a heads up.

[Bnyro]

Code lgtm and didn't find any issues when testing.
Well done!

[Bnyro]

(only resolved the conflicts, you can merge it if you want)