Fixed getCurrentUserName() method always returning "Owner" instead of current.

Method was returning name of the user in index 0.
Now it finds the current foreground user and returns its name.

Bug: 203422515
Test: atest com.android.systemui.statusbar.policy.UserSwitcherControllerTest#test_getCurrentUserName_shouldReturnNameOfTheCurrentUser
Change-Id: I8b94fd1394227e6f1858d6903837f7446a6771e1