Refactored start user flow in case of immediate restart after stop

Refactored start user to pause the start of a user if SHUTDOWN flow is
still running. The user's start will be resumed after the finish of
SHUTDOWN flow.

Improved Logging

Test: atest UserControllerTest -c
Test: atest UserManagerTest -c

Bug: 234166597
Change-Id: Ia292e3f6fb60c523c7e8a06a08c35151f6683e7a