New Sample: todo-mvvm-live-ktx

I'd like to propose a new sample using [Kotlin Android Extension View Binding](https://kotlinlang.org/docs/tutorials/android-plugin.html). Effectively this allows us to get rid of [Data Binding](https://developer.android.com/topic/libraries/data-binding) without the overhead of `findViewById()`.

This sample is based off of the [todo-mvvm-live-kotlin](https://github.com/googlesamples/android-architecture/tree/todo-mvvm-live-kotlin/) sample. My branch can be found here: https://github.com/agodding/android-architecture/tree/todo-mvvm-live-ktx.

The diff with `todo-mvvm-live-kotlin` branch: https://github.com/googlesamples/android-architecture/compare/todo-mvvm-live-kotlin...agodding:todo-mvvm-live-ktx?expand=1

Start by comparing the todo-mvvm-live-kotlin sample with the proposed todo-mvvm-live-ktx branch and its linked diff. Check how the sample replaces Data Binding with Kotlin Android Extension View Binding; done means the new sample is added consistently with the repository's existing sample structure.

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