Add NetworkRequest.Builder creating from an existing instance

Provide a formal way to construct a new NetworkRequest from an
existing instance. The network capabilities inside the
NetworkRequest is hidden. There is no way to pass a
NetworkRequest and update its capabilities. Add
NetworkRequest.Builder creating from an existing instance to
allow to clone the network capabilities.

Bug: 172183305
Test: make update-api
Change-Id: I068462b2a1410daf67b0c95f2b643d396f079531
CTS-Coverage-Bug: 172183305

framework/api/current.txt

@@ -396,6 +396,7 @@ package android.net {
 
   public static class NetworkRequest.Builder {
     ctor public NetworkRequest.Builder();
+    ctor public NetworkRequest.Builder(@NonNull android.net.NetworkRequest);
     method public android.net.NetworkRequest.Builder addCapability(int);
     method public android.net.NetworkRequest.Builder addTransportType(int);
     method public android.net.NetworkRequest build();

framework/src/android/net/NetworkRequest.java

@@ -215,6 +215,14 @@ public class NetworkRequest implements Parcelable {
             mNetworkCapabilities.setSingleUid(Process.myUid());
         }
 
+        /**
+         * Creates a new Builder of NetworkRequest from an existing instance.
+         */
+        public Builder(@NonNull final NetworkRequest request) {
+            Objects.requireNonNull(request);
+            mNetworkCapabilities = request.networkCapabilities;
+        }
+
         /**
          * Build {@link NetworkRequest} give the current set of capabilities.
          */