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