Migrate UserSwitcherScreenshotTest to Kosmos.

TODO: Replace mock() and inline constructor with more Kosmos.

Bug: 310729135
Test: SystemUIGoogleRoboRNGTests, SystemUIGoogleScreenshotTests
Flag: None
Change-Id: I47f2444e033aa5e9b25a35ed6893429a1018d89f
7 files changed