Prevent ephemeral user from being re-entered after stop.

Once the ephemeral user stops, the user's deletion is scheduled.
It takes a while before the user actually disappears and it is not
desirable for the user to be re-entered in the meantime.
Mark the user as disabled on stop and check this flag
in the activity manager to prevent the user from being switched
to again. Also hide the user from user-switching UI.

BUG: 26795729
BUG: 26780152

Change-Id: I83a61674958954b5a210114b88ffa5ae55922c1f