* Keep track of the tab-pickup-container views in TabsSetupFlowManager and their visibility * Fix visibilitychange handling in tab-pickup-container and add some tests * Capture a timestamp when a device has been added and there are 0 tabs to show, with at least one visible tab-pickup-container view * Record telemetry when there are > 0 tabs in these conditions * Small change to rename the `_waitingForTabs` internal tracking property to `_waitingForNextTabSync` to better clarify its use and meaning * Use a consistent pattern in some of the existing tests with how we mock SyncedTabs.getRecentTabs Differential Revision: https://phabricator.services.mozilla.com/D170526