[Flexiglass] Fix the password bouncer alignment issue

Align the password bouncer in the center vertically in
BesideUserSwitcherLayout. Updated screenshots in the bug.

Bug: 413303536
Test: Enable flexiglass and verify the bouncer on desktop.
Flag: com.android.systemui.scene_container

Change-Id: Ib2f80902277389ec4cb6d000cbf5d26180e98ecf