Environment

• CLI: 6.3.3
• Cross-platform modules: 6.3.0
• Android Runtime: 6.3.1
• iOS Runtime: 6.3.0
• Plugin(s): None

Describe the bug
When setting a selectedIndex on the Tabs tag as per https://docs.nativescript.org/ui/components/tabs, it is only respected by the TabContentItem and not the TabStrip and TabStripItem - these default to the first item.

Also, when returning to the page containing the Tabs using goBack, the TabContentItem is correct, but again the TabStripItem is not set - this would appear to be related.

To Reproduce
Run a simple tabs example and set the selectedIndex

Expected behavior
The TabStripItem and TabContentItem should respect the selectedIndex