Add account capability for undo.

If undo isn't supported, never show undo bar.

Change-Id: I061334e2e9e2bf9834dd5eb4ac94b479877adc21
diff --git a/src/com/android/mail/providers/UIProvider.java b/src/com/android/mail/providers/UIProvider.java
index e97528f..2dd7477 100644
--- a/src/com/android/mail/providers/UIProvider.java
+++ b/src/com/android/mail/providers/UIProvider.java
@@ -176,6 +176,10 @@
          * in this application.)
          */
         public static final int MULTIPLE_FOLDERS_PER_CONV = 0x1000;
+        /**
+         * Whether the provider supports undoing operations. If it doesn't, never show the undo bar.
+         */
+        public static final int UNDO = 0x2000;
     }
 
     public static final class AccountColumns {
diff --git a/src/com/android/mail/ui/ConversationListFragment.java b/src/com/android/mail/ui/ConversationListFragment.java
index 7f5b86c..6315bb1 100644
--- a/src/com/android/mail/ui/ConversationListFragment.java
+++ b/src/com/android/mail/ui/ConversationListFragment.java
@@ -435,7 +435,7 @@
 
     @Override
     public void onUndoAvailable(UndoOperation op) {
-        if (op != null) {
+        if (op != null && mAccount.supportsCapability(UIProvider.AccountCapabilities.UNDO)) {
             if (mUndoView == null) {
                 mUndoView = (UndoBarView) mActivity.findViewById(R.id.undo_view);
             }