Added a new API for SUW: ActivityManager#switchUser(UserHandle).

The new API is a SystemApi that mirrors ActivityManager#switchUser(int).
It also requires the MANAGE_USERS permission.

Bug: 113841016
Test: atest com.android.server.pm.UserManagerTest#testSwitchUserByHandle
Test: atest com.android.server.pm.UserManagerTest#testSwitchUser
Test: manual (run setup wizard)
Change-Id: Ica59863b90226131259ae22a3980526f054fd543