Fix document stack.

We need to add document to stack by using 'push' method instead of 'add'
method.

BUG=26437613

Change-Id: I3e4ac6b0d03cacbe2d6cdd93443ce1c96c8c47d9
diff --git a/packages/DocumentsUI/src/com/android/documentsui/State.java b/packages/DocumentsUI/src/com/android/documentsui/State.java
index 14ff902..2f0224f 100644
--- a/packages/DocumentsUI/src/com/android/documentsui/State.java
+++ b/packages/DocumentsUI/src/com/android/documentsui/State.java
@@ -105,7 +105,7 @@
     }
 
     public void pushDocument(DocumentInfo info) {
-        stack.add(info);
+        stack.push(info);
         mStackTouched = true;
     }