fix(ios): initialContext not always set before name

[Steven0351]

fix(ios): Application of props is not guaranteed to be in any specific order. Setting initialContext may occur before the name prop is set and therefore will be unable to set any initialContext. To address this, we now store the initialContext on the view itself and then apply it to the portal just before rendering.

[Steven0351]

This is a bit of an issue for both iOS and Android, where we need to do manual housekeeping to make sure we can render with any potential overridden initialContext. I think in a future release it would make sense to make these into one prop instead of two separate values. This would make the callsite look like this:

<PortalView 
  portal={{ name: 'help', initialContext: { startingRoute: '/help' } }}
  style={{ flex: 1 }}
/>

instead of this:

<PortalView
  name='help'
  initialContext={{ startingRoute: '/help' }}
  style={{ flex: 1 }}
/>

This is more of an issue right now on iOS because setting initialContext is triggering the didSet method of the _portal variable at least twice in some cases, which adds and removes the PortalUIView each time. If (or when) Android has a View based PortalView, the same issue would likely arise there as well.