Improvements in UMS.isUserRunning()

Modified isUserRunning() to return false when user is in state of STOPPING or SHUTDOWN even if the user is present in mUserStates.

Defined a new STATE_NONE state in UserState to discard the use of "-1" where we want to return an unknown state when user is no more available.

Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
Test: atest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest

Bug: 244798930
Change-Id: If5b9098f88a1f58cc3f74d0067cbcce6aa1062a6