Fix UserController to not throw in stopUser

Return an USER_OP_* instead of throwing an IllegalException

Test: atest UserControllerTest#stopUser_invalidUser UserControllerTest#stopUser_systemUser UserControllerTest#stopUser_currentUser
Fixes: 259740023

Change-Id: I7e50412e028d225b7c97e113a3e4aac138da2314
2 files changed