Fix reply/forward to start the compose activity

The header views that started the compose activity were using
the application context, but they need the activity context to
push new activities onto that activity's back stack.

Change-Id: I55b3a19c32afb7215c162529f4d74f09ae25fc29
diff --git a/src/com/android/mail/ui/ConversationViewFragment.java b/src/com/android/mail/ui/ConversationViewFragment.java
index e271503..f9efa1c 100644
--- a/src/com/android/mail/ui/ConversationViewFragment.java
+++ b/src/com/android/mail/ui/ConversationViewFragment.java
@@ -208,7 +208,7 @@
         mWebView.loadDataWithBaseURL(mBaseUri, renderMessageBodies(messageCursor), "text/html",
                 "utf-8", null);
         mConversationContainer.setOverlayAdapter(
-                new MessageListAdapter(mContext, messageCursor, mAccount));
+                new MessageListAdapter(mActivity.getActivityContext(), messageCursor, mAccount));
     }
 
     @Override