Make Setting UI use system proxy validity check

Setting UI performs some validity check on proxy settings, such that user
can't click "Save" until inputs are valid. However, it uses a different
validity pattern than framework, such that user may still be able to click
"Save" but the changes are silently rejected by framework. Fixed by using
the system proxy validity check routine.

bug: 13248097
Change-Id: Ia1eef53588e6923a838cb5629e373697001a8ba6