[User Switcher] Remove duplicate views.

Ensures that there are no duplicate views for full screen user switcher.
Check the children for the grid container view instead of the root view
as the root view is not a direct parent of the user views.

Fixes: 257544527
Test: Open full screen user switcher. Using the power button, turn the
screen off and back on.

Change-Id: I32ae88550dc2d7e29acda08e2e9360c8590c5954