[Flexiglass] Fix the user switcher alignment on bouncer

Vertically align the user switcher in the center in
`BesideUserSwitcherLayout`.

Bug: 413302135
Test: Bring up the bouncer with flexiglass enabled in Android desktop,
observe that the user switcher is vertically aligned in the center.
Flag: com.android.systemui.scene_container

Change-Id: Ie19905a0a1c3432f7ba3dcc4b7e63117e544e2d2