Show "(no subject)" in conversation view header.

Fixes b/10299458.

Change-Id: I3fe3a610e7c40cef3c0597c8458bbd21da8ccd7f
diff --git a/src/com/android/mail/browse/ConversationViewHeader.java b/src/com/android/mail/browse/ConversationViewHeader.java
index 5a4b66c..7ca0bb8 100644
--- a/src/com/android/mail/browse/ConversationViewHeader.java
+++ b/src/com/android/mail/browse/ConversationViewHeader.java
@@ -138,9 +138,10 @@
     }
 
     public void setSubject(final String subject) {
-        mSubjectView.setText(subject);
         if (TextUtils.isEmpty(subject)) {
-            mSubjectView.setVisibility(GONE);
+            mSubjectView.setText(R.string.no_subject);
+        } else {
+            mSubjectView.setText(subject);
         }
     }