ConversationFragment: enable back button to dismiss

The ConversationFragment has a AlertDialog for showing the message
details, which sets the cancelable property to be false. This stops the
user from being able to use the back button to dismiss the dialog.

src/org/thoughtcrime/securesms/ConversationFragment.java

@@ -153,7 +153,7 @@ public class ConversationFragment extends SherlockListFragment
     AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
     builder.setTitle(R.string.ConversationFragment_message_details);
     builder.setIcon(Dialogs.resolveIcon(getActivity(), R.attr.dialog_info_icon));
-    builder.setCancelable(false);
+    builder.setCancelable(true);
 
     if (dateReceived == dateSent || message.isOutgoing()) {
       builder.setMessage(String.format(getSherlockActivity()