Bug 990035 - Modify phone-sized tutorial to show vertical-scroll, notifications & sheets video. r=borjasalguero

* modify phone-layout tutorial config to use new video assets and tutorial strings
* placeholder video assets
* add optional callback to _setStep and handle image/video load errors
* hide overflow in tutorial step image/video container
* fix ui test for 3 steps
* remove orphaned tutorial images
* use .step-video-base to ensure video is height:100%
* Post-backout: Refactor tests and clarify expectation in event of config load failure
Merge pull request #20164 from sfoster/bug-990035-video-tutorial

Bug 990035 - Modify phone-sized tutorial to show videsteps. r=borjasalguero