Add ActivityManager.logoutUser() hidden API

This logoutUser switches to an appropriate user, either
headless system user or previous foreground user.
It currently does not prevent another user from logging in, before the
current user is fully stopped and credential key evicted.

Bug: 380125011
Flag: android.multiuser.logout_user_api
Test: m
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
Test: atest FrameworksMockingServicesTests:com.android.server.am.UserControllerMockedTest

Change-Id: I2449888e71ce667352d442b93b37b96c4f310735