message view: fix subject bugs

- Android does not support ellipsize in combination with maxlines
  for TextViews. This caused getEllipsisCount() in MessageTitleView
  to always fail, and the full subject was never shown in the regular
  headers area when needed.
  To work around that, check for ourselves whether the text is
  longer than 2, and ellipsize manually.

- Clicking the star button on a message caused the subject line to
  re-appear, even if it fits in the action bar title without being
  cut off. This was caused by MessageHeader.populate(), which always
  set the subject to visible.
  As a workaround: Only set subject to visible in case populate()
  actually shows a new message.

- delete res/layout/actionbar_message_view.xml, its already present
  in the actionbar_custom.xml