Add the margin between user pods.

In fullscreen user switcher, there is a margin that needs to be
accounted for between user pods.  This is not the same as the margin
that increases the size of a user pod.

Also fixes the way the layout is done so that it handles long user
names properly with ellipsis.

Bug: 36454400
Tested on Mojave

Change-Id: Idb3468f6a8c3f1b81a96c5ff73ff1d32c48e2c82