Listen for changes in the Contacts database with RxJava/RxAndroid

[vestrel00]

Depending on API design, the core APIs may need to be modified in order to support this.

Think of a way to do this without using RxJava/RxAndroid. In other words, support this functionality in the core module without depending on any frameworks. The rx extensions should just be that, extensions.

Depending on the amount of work to be done here, it may be best to split this into separate issues once we've determined what to do.