Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: next
• Cross-platform modules: next
• Android Runtime: next
• iOS Runtime: next

Describe the bug
When you call ObservableArray.splice(), the event it emits fails to account for replaced items. This means it accounts only for removals/inserts, but any replace returns an added count of 0

To Reproduce
https://play.nativescript.org/?template=play-ng&id=jVKVft

Expected behavior
addedCount should take into account the replaced items

Additional context
Maybe we should change addedCount to this._array.length + removed.length - oldLength

I ran into this issue trying to splice FormattedString.spans. The only way I could make it work was by either:

1. removing and adding again
2. applying the addedCount fix I proposed

Since ObservableArray is used almost everywhere, this change could be breaking.