Move shared Proxy method and constants to shared lib

The static validate() method an PROXY_* constants in Proxy are
used by both module and Settings. After Proxy is moving into
connectivity module, Setting will not able to access them.
Thus, move them to ProxyUtils in net shared lib.

Bug: 172183305
Test: atest FrameworksNetTests
Test: atest CtsNetTestCases:ProxyInfoTest
Change-Id: I02c88aeaeb53d62b87a42a2d8ead9b140b054650