[flexiglass] Foldable UI support for bouncer.

A small tweak to get the foldable UI just right.

Because the user switcher UI shouldn't be available on non-tablet
devices like foldables, the presentation logic in the composable is
slightly changed.

Note that for the unfolded + landscape case we do want to use the side-by-side layout, but we want an
empty placeholder instead of the user switcher.

Fix: 299343639
Test: manually tested folded, unfolded + landscape, unfolded + portrait
on foldable device with pattern unlock
Test: manually tested tablet device in landscape and portrait, also with
pattern unlock

Change-Id: Ibe9611209ea9b2b4f03ecf72db72e7dbab88a072