Add instrumentation to test NetworkAgent.

Test: NetworkAgentTest, new tests using this API
Bug: 139268426
Change-Id: I0b65be788bb742fd1a8c0ca624e97368462f9b6a
Merged-In: Ia83b1c896df63bb18e2aa4b74d6cc09eba990eb5
(cherry picked from commit d89dcb9765b9c73c950661faaf8af9b795934acb, aosp/1284574)

core/java/android/net/NetworkAgent.java

@@ -25,12 +25,14 @@ import android.compat.annotation.UnsupportedAppUsage;
 import android.content.Context;
 import android.os.Build;
 import android.os.Bundle;
+import android.os.ConditionVariable;
 import android.os.Handler;
 import android.os.Looper;
 import android.os.Message;
 import android.os.Messenger;
 import android.util.Log;
 
+import com.android.internal.annotations.VisibleForTesting;
 import com.android.internal.util.AsyncChannel;
 import com.android.internal.util.Protocol;
 
@@ -571,6 +573,37 @@ public abstract class NetworkAgent {
         return mNetwork;
     }
 
+    /**
+     * Register this network agent with a testing harness.
+     *
+     * The returned Messenger sends messages to the Handler. This allows a test to send
+     * this object {@code CMD_*} messages as if they came from ConnectivityService, which
+     * is useful for testing the behavior.
+     *
+     * @hide
+     */
+    public Messenger registerForTest(final Network network) {
+        log("Registering NetworkAgent for test");
+        synchronized (mRegisterLock) {
+            mNetwork = network;
+            mInitialConfiguration = null;
+        }
+        return new Messenger(mHandler);
+    }
+
+    /**
+     * Waits for the handler to be idle.
+     * This is useful for testing, and has smaller scope than an accessor to mHandler.
+     * TODO : move the implementation in common library with the tests
+     * @hide
+     */
+    @VisibleForTesting
+    public boolean waitForIdle(final long timeoutMs) {
+        final ConditionVariable cv = new ConditionVariable(false);
+        mHandler.post(cv::open);
+        return cv.block(timeoutMs);
+    }
+
     /**
      * @return The Network associated with this agent, or null if it's not registered yet.
      */