Improved UserController so users cannot start before it's ready.

Currently, it's possible to crash the system by calling
`am start-user` before UserController is ready; this CL adds a
guard that prevents it.

Test: manual verification by manually calling SystemClock.sleep
Test: atest FrameworksServicesTests:UserControllerTest
Fix: 420514563
Flag: EXEMPT trivial bug fix

Change-Id: If05557c4ab188d3802780c1b30babe6632604dd0