logoutUser always switches to system user

Previously, logoutUser would switch to previous foreground user, because
of HSUM devices that can't switch to system user. Now, logoutUser no
longer supports such devices, so it's always switch to the system user:
both HSUM devices with interactive system user, and non-HSUM devices.

This allows to simplify logoutUser a lot, and remove a potential race
condition.

getUserToLogoutCurrentUserTo() and related tests are removed, as it is
no longer used.

Bug: 380125011
Flag: android.multiuser.logout_user_api
Test: atest FrameworksMockingServicesTests:com.android.server.am.UserControllerMockedTest
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceMockedTest
Test: Manual test. adb shell am switch-user 10 && adb shell am logout-user 11 && adb shell am switch-user 10 && adb shell am logout-user 10
Change-Id: I2472604ca7b50ff1362dfc7645e79e6d438d06f3