Do not stop lock task if it is starting background user

We do not switch user when starting background user, so no need to
stop lock task.

Refactor a bit to make unit test possible.

Test: runtest -x frameworks/base/services/tests/servicestests/src/com/android/server/am/UserControllerTest.java
Test: Lock an app using TestDPC, creating work profile, lock task is not dismissed

Bug: 35591838

Change-Id: I52c1f6ba07860df9266462750bf953a002f69026