Update the WidgetPickerActivity to display recommendations for hub host

- Accepts a ui_surface param of format "widgets{_hub}" and existing
widgets on the surface to be excluded from predictions
- Refactored the widgets prediction update task to extract reusable
logic that maps the predictions to widget items and reused it.

http://screencast/cast/NjE1MTA5MDI0NzU2NTMxMnwzMGE3NTMwNi1hZg

Bug: 326092660
Test: WidgetsPredictionHelperTest and see screencast above.
Flag: N/A
Change-Id: I6ceeb752c167893bab4ed496cedc5e8081e1b950