MessageView: Theme fix when global and message view theme differ

- Since the split-view change, MessageView is only a fragment, so we
  can't call setTheme() anymore so easily.
  Instead, use a ContextThemeWrapper and use that to inflate the
  layout. This way the message header and attachment view
  are styled correctly.

- The HTC WebView fix in SingleMessageView was returning the wrong
  background color, when message view theme and global theme differ,
  because it always used the global theme to retrieve it.
  Fix: Specifically put the light/dark values in the themes.xml,
  and get them using getContext().getTheme().resolveAttribute().
  getContext() will use the ContextThemeWrapper from above, so
  even if the global and message view themes differ, it aleays
  returns the correct one.
  The getThemeBackgroundColor() method added to the K9ActivityMagic
  interface in 309eeb72ac3c4fb2b93766550eed24e5cab39bb8 is now not
  needed anymore, and was removed.