userportal, webadmin: fixed unchecked cast to PropertyChangedEventArgs

fixed unchecked cast to PropertyChangedEventArgs in AbstractLoginPresenterWidget.java.
Fixes a Coverity complaint.

Change-Id: I5f0d08647d3fed033fa66f6ed8351d83e95680c0
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>