Mechanism for overriding notification behavior

[c-b-h]

onDistinctItems is called when the adapter receives new (distinct from previous) items. This allows for changing the notification behavior in a backward-compatible way e.g. from notifyDataSetChanged() to notifyItemRangeInserted()`.

Our app uses a custom ItemAnimator for insertions and with this change, we're able to call notifyItemRangeInserted when the adapter initially receives items and there by trigger an insert animation as can be observed in this clip:

https://user-images.githubusercontent.com/14194998/116545693-033f5900-a8f1-11eb-9de5-df6c71cc59ed.mov

The current behavior is that notifyDataSetChanged() is called which for the first item will not trigger an insert animation. See clip for difference and notice the first item:

https://user-images.githubusercontent.com/14194998/116545869-3c77c900-a8f1-11eb-9412-8f3e7135ef58.mov