Turns on refactored UserSwitcherController.

Now that we fixed all bugs discovered by QA, we can turn this on for
droidfooders to see.

Bug: 246631653
Test: Manually made sure that the new implementation is being hit when I
attempted to add a new user on a phone

Change-Id: I9fcbfb16f9468afdf486e597c36417162c5388f1