[flexiglass] Bouncer scene large screen support.

There are three different layouts in bouncer scene now:
- Large screens, landscape orientation: side-by-side layout where one
  side is the bouncer contents (where the user enters PIN, password, or
  pattern) and the other is the user switcher. Double tapping on the
  side opposite of the bouncer contents switches the two sides to bring
  the bouncer contents closer to the side that was double tapped.
- Large screens, portrait orientation: stacked layout where the user
  switcher is on top and the bouncer contents are below.
- Non-large screens: just the bouncer contents, just like before.

In this CL, the user switcher is just a placeholder. Will add it in
followup CLs.

Bug: 299343639
Test: please see screenshots and video on the associated bug.
Change-Id: I975e63c2b94ddf006836159befb3242187bbd40b