Add IpConfiguration parcelable interface implementation to system API

API lint robot warnings that parcelable requires writeToParcel
and describeContents.

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: I7acf000c468788c3fdcb47f8601762e7804ab940

core/java/android/net/IpConfiguration.java

@@ -191,18 +191,12 @@ public final class IpConfiguration implements Parcelable {
                83 * httpProxy.hashCode();
     }
 
-    /**
-     * Implement the Parcelable interface
-     * @hide
-     */
+    /** Implement the Parcelable interface */
     public int describeContents() {
         return 0;
     }
 
-    /**
-     * Implement the Parcelable interface
-     * @hide
-     */
+    /** Implement the Parcelable interface */
     public void writeToParcel(@NonNull Parcel dest, int flags) {
         dest.writeString(ipAssignment.name());
         dest.writeString(proxySettings.name());