Fix the failing DocumentsUIGoogleTests:UserManagerStateTest

Add the class to proguard.flags following the guide on go/xf-docsui#nosuchmethoderror-when-running-tests

Bug: 428927352
Test: atest DocumentsUIGoogleTests
Flag: android.multiuser.enable_moving_content_into_private_space
Change-Id: I571b85d3ec4cc7059a8461f9670a4dd2ad36adb6