Add request for a11y focus for item

In fullscreen user switcher, when we select add, the first item on the
list is not focused in talkback. Upon inflation, we add a 200ms delayed
a11y request.

Fixes: 273874539
Test: enable talkback, go to fullscreen user switcher and go to add. The
first item "Add User" is highlighted and announced.

Change-Id: I919160ed0a03f100d1382b1da76ca2e8c2658fcb