Updated Fullscreen user switcher to not use fitsSystemWindows.

fitsSystemWindows="true" has default behavior of setting padding of the
view to ensure contents don't overlay system windows. In the case of the
fullscreen user switcher, we don't need this behavior.

Bug: 150302361
Test: Manual (set lockscreen, restart, click "Cancel")
Change-Id: Ifc597c2b96862abc3283d6af1a0c2765431c71af