Makes Activity.addDumpable() a @TestApi

Test: atest CtsAppTestCases:android.app.cts.ActivityDumpTest
Test: m update-api

Bug: 149254050
Bug: 215435293

Change-Id: I3f570e2cc107a6924bbe71dbc47e0508a3b8cb38

core/api/test-current.txt

@@ -103,6 +103,7 @@ package android.animation {
 package android.app {
 
   @UiContext public class Activity extends android.view.ContextThemeWrapper implements android.content.ComponentCallbacks2 android.view.KeyEvent.Callback android.view.LayoutInflater.Factory2 android.view.OnBackInvokedDispatcherOwner android.view.View.OnCreateContextMenuListener android.view.Window.Callback {
+    method public final boolean addDumpable(@NonNull android.util.Dumpable);
     method public void onMovedToDisplay(int, android.content.res.Configuration);
   }
 

core/java/android/app/Activity.java

@@ -7098,6 +7098,7 @@ public class Activity extends ContextThemeWrapper
      * @hide
      */
     @SystemApi(client = SystemApi.Client.MODULE_LIBRARIES)
+    @TestApi
     public final boolean addDumpable(@NonNull Dumpable dumpable) {
         if (mDumpableContainer == null) {
             mDumpableContainer = new DumpableContainerImpl();