Clean up public surface of ReactNative library

[rozele]

This includes sealing classes as appropriate, and making anything that does not need to be public internal.

[rozele]

Let's have another pass over things like ReactInstanceManager and ReactPage. Make things fully async where they should be. Having async void bit us on a couple unit tests and I can see it doing the same to power users who want to set up their own ReactInstanceManager embedded within a UWP app.

[rozele]

Should be done as part of the work to refactor into a collection of .NET Standard libraries, but this issue is too large to be actionable.