Don't call NotifyDataSetChanged for count changes.

Removed redundant call to NotifyDataSetChanged in BaseModelsViewAdapter
as when the Count changes the Models property is also changed, thus
currently we invalidated the data twice, for no benefit.
Workaround mono bug with losing context.

For some reason mono lost the actual object context in the event
handler, meaning the NotifyDataSetChanged would throw a weird exception.
Adding the Handle check preserves that context.