fix(menu): menu focus is lost when menu-item has custom content
fix(menu): menu focus is lost when menu-item has custom content

- and the first child of the `md-menu-item` is a disabled custom element
- check that the firstElementChild doesn't have the disabled attribute set as well
- this supports `ng-disabled` and `disabled` on a custom element

Fixes #12054

angular/material,oostmeijer/material,oostmeijer/material,angular/material,angular/material,angular/material,oostmeijer/material,oostmeijer/material