MessageView: another theme fix

- The attachments view still had the wrong background color in case
  of different global and message themes.

- The attachments view used the activity LayoutInflater, but it needs
  to use the one of the fragment.

- The background drawable for the attachments used transparency, and
  thus was completely invisible in the black theme. Fix it by adding
  another one for the black theme.
MessageView: fix crash when clicking on URLs in WebView

The ContextThemeWrapper added in a74d57cb71d67e42e1db709e00d5fd514b1ecd0c
used getActivity().getApplicationContext() to get the base context.
This is wrong, because an Application context won't work for starting
activities from the WebView.

Instead, use the context that is given to us in the constructor as base
context. This is the one that would also  be used if no ContextThemeWrapper
was present at all.
Remove old subject from action bar when loading new message
Restructure MessageHeader to avoid NPE when used by MessageTitleView
Fix NullPointerException
Don't show animation when directly opening message view
Only remove message view fragment after animation is complete