Reduce boilerplate in each Activity in the way the ViewModel is obtained

[CDRussell]

Description:
No Asana issue for this one, as it's just a code tidy up.

Although I haven't functionally changed anything, this PR reduces the boilerplate needed in each Activity to obtain its ViewModel. We do this by moving the worst of the boilerplate to the parent Activity as a function.

The result now is that in each Activity, obtaining the ViewModel is as simple as

Now 👇

private val viewModel: BookmarksViewModel by bindViewModel()

Before 👇

@Inject	
lateinit var viewModelFactory: ViewModelFactory

private val viewModel: BookmarksViewModel by lazy { 
    private val viewModel: BookmarksViewModel by bindViewModel()
    ViewModelProviders.of(this, viewModelFactory).get(BookmarksViewModel::class.java)	
}

Steps to test this PR:

1. Just make sure the app is compiled and opens without crashing

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Internal references:

Software Engineering Expectations
Technical Design Template

[brindy]

Definitely an improvement.

I strongly dislike the ViewModelFactory though, would much prefer to have a module for view models and inject them directly.

That said, ultimately we'd get rid of Dagger as well. :)