DocumentsUI: Use system cancel string resources.

BUG=None

Change-Id: I989e28ad4729176d66b1b333f242470129d139fa
diff --git a/packages/DocumentsUI/src/com/android/documentsui/CopyService.java b/packages/DocumentsUI/src/com/android/documentsui/CopyService.java
index 337b862..d2ef3d7 100644
--- a/packages/DocumentsUI/src/com/android/documentsui/CopyService.java
+++ b/packages/DocumentsUI/src/com/android/documentsui/CopyService.java
@@ -190,7 +190,7 @@
         final Intent cancelIntent = new Intent(this, CopyService.class);
         cancelIntent.putExtra(EXTRA_CANCEL, mJobId);
         mProgressBuilder.addAction(R.drawable.ic_cab_cancel,
-                getString(R.string.cancel), PendingIntent.getService(this, 0,
+                getString(android.R.string.cancel), PendingIntent.getService(this, 0,
                         cancelIntent, PendingIntent.FLAG_ONE_SHOT));
 
         // Send an initial progress notification.