Move keyguard multi-user icon into container view

Fix UI bug that made the lock screen unusable when toggling the
multi-user setting off and on.

Test: adb shell settings put global user_switcher_enabled 0 \
        && adb shell settings put global user_switcher_enabled 1
Bug: 195052728
Change-Id: Iadbd29bd7c50cd20a86f966d3f7ec69cd78c9744