Make it possible to remove current user

Due to the async nature of switching users, it's not possible to
switch and remove immediately. So mark the switch target user as
soon as the user switch is requested, so that a remove will proceed
without failing at stopUserLocked().

Also, fix a similar problem with deleting the current guest and
switching to a new guest. It was attempting to remove the current
user which will result in a failed stopping of the user.
Added a way to mark the current guest
for deletion so that a new one can be created, switched to and the
old one deleted. If runtime fails, old guest is already marked for
deletion and will be cleaned up on restart.

Bug: 17321533
Change-Id: I4be0fb956227e0bb95588b5b1f2867fb1e655c0d