Replace the hidden ProxyInfo methods usage

The hidden ProxyInfo methods usage should be replaced because
ProxyInfo is moving to connectivity mainline module. Setting
will not be able to access it. Replace the usage with
corresponding APIs.

Bug: 172183305
Test: make RunSettingsRoboTests ROBOTEST_FILTER=\
      com.android.settings.wifi.WifiConfigControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=\
      com.android.settings.wifi.WifiConfigController2Test
Test: manually update proxy from setting and check the result
Change-Id: I59192d0d5d38c833eb83cc930e358a738ebe3d13