Show current user first in KeyguardUserSwitcher

Update Adapter in KeyguardUserSwitcher so that the current user is
always the first entry in the list.

Bug: 169783558
Test: Open keyguard user switcher as secondary user, check that current
      user is the first item in the list
Test: atest KeyguardUserSwitcherAdapterTest
Change-Id: I14ec36cbbfe908989f7a30e5c28f3b7badfb6ca2