Only animate user list if there's multiple users

Prevent ArrayIndexOutOfBoundsException from occurring if there are no
child views yet.

Test: Enable keyguard user switcher and reboot several times
Bug: 198642878
Change-Id: I4a70e1bb93652996764a533b2c84241653c007da
Merged-In: I4a70e1bb93652996764a533b2c84241653c007da