Add @TestApi for the methods of VoiceInteractionSession.ActivityId

In order to verify which activity has become visible or
invisible, we need to add @TestApi for the methods of
VoiceInteractionSession.ActivityId class to get activity
data.

Bug: 246456675
Bug: 235296082
Test: atest VoiceInteractionSessionVisibleActivityTest
Change-Id: Ic659f23b7681a91d070cc841c8f7503b84b78092

core/api/test-current.txt

@@ -2481,6 +2481,11 @@ package android.service.voice {
     ctor public VisibleActivityInfo(int, @NonNull android.os.IBinder);
   }
 
+  public static class VoiceInteractionSession.ActivityId {
+    method @NonNull public android.os.IBinder getAssistToken();
+    method public int getTaskId();
+  }
+
 }
 
 package android.service.watchdog {

core/java/android/service/voice/VoiceInteractionSession.java

@@ -24,6 +24,7 @@ import android.annotation.IntDef;
 import android.annotation.IntRange;
 import android.annotation.NonNull;
 import android.annotation.Nullable;
+import android.annotation.TestApi;
 import android.app.Activity;
 import android.app.Dialog;
 import android.app.DirectAction;
@@ -2311,11 +2312,15 @@ public class VoiceInteractionSession implements KeyEvent.Callback, ComponentCall
             mAssistToken = assistToken;
         }
 
-        int getTaskId() {
+        /** @hide */
+        @TestApi
+        public int getTaskId() {
             return mTaskId;
         }
 
-        IBinder getAssistToken() {
+        /** @hide */
+        @TestApi
+        @NonNull public IBinder getAssistToken() {
             return mAssistToken;
         }