Add interface to pass root tag to IViewManager

[rozele]

Proposal: Add interface to pass root tag to IViewManager

Summary

The UIManager module sends the rootTag to the createView native method. For view managers like FlyoutViewManager, we could use this rootTag value to look up the appropriate XamlRoot when creating XamlRoot dependent views like flyouts.

We might want to add something like NativeUIManager::tryGetXamlRoot(int rootTag) as well.

Motivation

Currently, FlyoutViewManager gets the XamlRoot for the first root view created with a XamlRoot. This is error prone in multi-window apps, where we need to set the correct XamlRoot for views like flyouts.

[asklar]

Luckily, there's already a CreateViewWithProperties which would work for this :)

https://microsoft.github.io/react-native-windows/docs/IViewManagerCreateWithProperties

[asklar]

Another possibility: is the root tag associated with the react context 1:1? if so, we have IViewManagerWithReactContext

[chrisglein]

Problem with using CreateViewWithProperties is that rootTag would become a reserved word as this is going in alongside the JS properties.

Needs some design here. Eric's suggestion above may be just fine.