Small bug fixes to ReactiveRecyclerViewAdapter and ViewHolder

[Qonstrukt]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bug fixes for ReactiveRecyclerViewAdapter and ReactiveRecyclerViewViewHolder.

What is the current behavior? (You can also link to an open issue here)
Binding an inherited type of ViewModel to a ViewHolder doesn't currently raise change notifications, and you aren't able to use any other type of ViewHolder than ReactiveRecyclerViewViewHolder, while the adapter should only depend on IViewFor to work.

What is the new behavior (if this is a feature change)?
Properly call for this.RaiseAndSetIfChanged() when setting a ViewModel.
Allowing the use of any type of RecyclerView.ViewHolder which complies to IViewFor.

Does this PR introduce a breaking change?
Only if someone doesn't expect the change notices to happen, but that's clearly not the way it should work.

Please check if the PR fulfills these requirements

• The commit follows our guidelines: https://github.com/reactiveui/reactiveui#contribute
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
I was bored, so I made a PR. ™️

[ghuntley]

Thanks for the PR @Qonstrukt, could @reactiveui/reviewers-android please review.