Expose IpConfiguration and ProxyInfo APIs

Moving IpConfiguration methods to system API
for mainline support.

Public copy constructors of ProxyInfo and add
buildPacProxy(Uri, int) to create a new proxy
properties.

Bug: 139268426
Bug: 135998869
Test: atest android.net.cts
      atest android.net.wifi.cts
      atest FrameworksNetTests
      ./frameworks/opt/net/wifi/tests/wifitests/runtests.sh

Change-Id: I07bcd2a34a222ea2b3cf0d8b497f051011c41c2c