Tracking beginning and end of DOM changing for testability

Adding a window.tsbUpdatingUsers variable to track when does
JavaScript begin and end running API requests and changing the DOM.

This is done to facilitate testing changing elements using
Watir::Wait.while { @browser.execute_script "return window.tsbUpdatingUsers" }

Bug: 58742
Change-Id: I4db63906b240ce71503fd2bb1ac579b5c221ce9f