Annotation views are inaccessible to VoiceOver

[1ec5]

As of #4801, annotations can be presented as either GL annotations or annotation views. Whereas MGLMapView synthesizes an accessibility element for each GL annotation, it doesn’t do so for annotation views. MGLMapView should include annotation views as they are in its child accessibility elements.

/cc @boundsj

[1ec5]

It turns out that view-backed annotations are accessible, but they get the default frame, as if the view is the same size as the default red pin image. Anything inside the view is inaccessible.