Replace Proxy hidden constants and methods usages

Proxy is a part of incoming connectivity mainline module. The
hidden APIs are no longer available outside the module. These
shared constants and methods are moving to ProxyUtils insdide
net shared lib. Update the corresponding usage in the caller
side.

Bug: 172183305
Test: atest SettingsRoboTests
Change-Id: I2d51442e65dbcdf1a36940d8a25c7571363d56e6