UI screenshot diff test for user switcher.

Using the new, modern architecture user switcher UI, we implement a UI
screenshot diff test for it. This is a prerequisite for the next step,
where we replace the XML file and view-binder code with a Jetpack
Compose implementation.

Bug: 243843440
Test: N/A this CL is a test
Change-Id: I2831129f44702007be64133a3edcd418e30bd4c1