Fix the multi-user keyguard dialog animation

This CL fixes the animation that is run when clicking the user switcher
avatar on the keyguard, which is then expanded into a dialog.

See b/225000960#comment10 for before/after videos.

Fixes: 225000960
Bug: 225000960
Test: Manual
Change-Id: I1a97ddf9c2ba9387b7ccccb14eb7e5e83e089dec