Widgets: Tracking improvements

[staskus]

All types of lock screen widget tracking updates are reporting identical numbers. It's likely caused by all the update methods all called at once, rather than only for widgets that are added.

1. Investigate tracking behavior for lock screen widgets
2. Consider using Lock Screen Widgets: Add Tracks #21533 (review)

Steps to reproduce the behavior

Tested on [device], iOS [version], Jetpack iOS / WordPress iOS [version]