Fix up user switch flow a little.

- Remove the long click listeners since the deletion flow wasn't
  very discoverable or good anyway. Deletion is better accomplished
  in settings.
- The lock screen shows up immediately after user switch and gets
  called to be shown quite a few times in not very deterministic
  order. Now, we replace the user grid view with an infinite progress
  bar while the switch is in progress so it looks more natural.
- Switch the background to solid black like in the mocks.

Note that this does not fix the case of the lock screen showing up
briefly when switching using the notification shade. That will be
cleaned up when we replace that view with the UserGridView.

Test: Rebooted and verified lock screen still works fine.
Bug: 36454400
Change-Id: I19ad8e9305d93a069294e910998749f1f0386b5b