Demonstrate bug in differentiator where reparented nodes reference no…

…n-existent nodes

Summary:
Demonstrates the issue identified in facebook#47960 and a crash we've been seeing internally around `getViewState` referencing a view that does not exist.

When reparenting unflattened nodes, Differentiator may emit an `update` with a `parentShadowView` that does not exist on the native side yet, thereby crashing Android.

Landing the test-case first (with some test cleanup), so the diff for the actual fix is clearer.

Changelog: [Internal]

Differential Revision: D66557919