Add notification extensions for android wear

This adds two things to notifications on Android wear devices (currently only for single thread notifications):

1. An additional page lists all unread messages with sender and without truncation. This only applies if more than one message is unread.
2. A reply action to input a message by voice or choose from some predefined answers. For the predefined answers I chose the first couple of things that came to mind, so discussion is welcome.

I more or less copy-pasted the message sending part from the ConversationActivity, maybe this functionality should be refactored into a separate class.