Refactor android.backup => android.app.backup

Change-Id: I0b21316ff890d7f3c7d4b82837bb60670724c2e8
diff --git a/core/java/android/app/ActivityThread.java b/core/java/android/app/ActivityThread.java
index 0756c71..7c49bb7 100644
--- a/core/java/android/app/ActivityThread.java
+++ b/core/java/android/app/ActivityThread.java
@@ -16,6 +16,7 @@
 
 package android.app;
 
+import android.app.backup.BackupAgent;
 import android.content.BroadcastReceiver;
 import android.content.ComponentCallbacks;
 import android.content.ComponentName;