Merge pull request #22481 from ferjm/bug1044060.mobileiduitest

Bug 1044060 - [MobileID] Add Mobile ID test to Gaia UI test app. r=timdream
(cherry picked from commit d6c7edf333fab56210348bb7471d0c5a7b57497e)
Bug 1057604 - Use the hidden attribute for the save button r=dkuo

While title-player and title-done did proper use of their "hidden"
attribute to be hidden via CSS rule setting "display: none", the save
button was not part of this list. Thus, setting the "hidden" attribute
to true had no impact on the visibility of the element.