Skip disabled users in UserManager.getPreviousForegroundUser() method.

When a guest user is reset in Settings, UM.markGuestForDeletion() is
called for the previous guest user and it gets marked with
FLAG_DISABLED. After the reset, if
UserManager.getPreviousForegroundUser() is called, it returns that
previous guest user which is marked as disabled, but it shouldn't.
This CL adds an additional userData.info.isEnabled() check to skip the
disabled users.

Bug: 283106632
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
Test: atest UserManagerServiceCreateProfileTest
Test: atest UserManagerServiceIdRecyclingTest
Change-Id: I544b96a7fd42a9542862eb804a5557f19f5a4104