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

framework/src/android/net/ProxyInfo.java

@@ -23,6 +23,8 @@ import android.os.Parcel;
 import android.os.Parcelable;
 import android.text.TextUtils;
 
+import com.android.net.module.util.ProxyUtils;
+
 import java.net.InetSocketAddress;
 import java.net.URLConnection;
 import java.util.List;
@@ -233,7 +235,7 @@ public class ProxyInfo implements Parcelable {
      */
     public boolean isValid() {
         if (!Uri.EMPTY.equals(mPacFileUrl)) return true;
-        return Proxy.PROXY_VALID == Proxy.validate(mHost == null ? "" : mHost,
+        return ProxyUtils.PROXY_VALID == ProxyUtils.validate(mHost == null ? "" : mHost,
                 mPort == 0 ? "" : Integer.toString(mPort),
                 mExclusionList == null ? "" : mExclusionList);
     }