Add public ViewManagerModel.viewSelectionObserver

[amcclain]

• For use by apps to mask components during view select/rebuild.

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[amcclain]

Testing out our new stuff in an actual app that manages a fair amount of state via viewManager - on switch there's a noticeable lag/pause while the components are all given their new state. Exposing this task observer allowed me to put a mask in place, which I think works well to let the user know what's happening.

[amcclain]

@ghsolomon rightly pointed out that this random 100 value is unfortunate. I re-tested, and I found that I was unable to get the mask to reliably render in my app when swapping views unless I left this "high" value in place. await wait() and await wait(1) did not do it.

(The app test case involve some heavyweight re-filtering of a large recordset + dashboard layout changes, but nothing that extreme - definitely a "real world" case.)

We discussed how ideally we could use some utility that perhaps keyed off of animation frames or otherwise was able to better detect that rendering had "settled down" - for want of a more technical term...