UserSwitcher in Compose.

Adds a screen to the gallery app to test it. Did not end up using the UI
screenshot diff test because the buttons are not the same in Compose as
they are in this bespoke UI. The UI is simple enough that I was able to
get close enough.

Fix: 246002091
Test: In the next CL, a screen is added to the gallery app to test it.
Change-Id: I90d871bb819ecd7bd75a3f55746a94f7a37f68c2