store contact id in messages table instead of contact name string; merge...

store contact id in messages table instead of contact name string; merge messages and group messages tables.

- removed GroupMessages API, groupchat messages are merged with messages. So where we used to have
  GroupMessages.CONTENT_URI, now we have Messages.GROUP_MESSAGES_CONTENT_URI. Messages api now has a flag
  IS_GROUP_CHAT to indicate that a message is a groupchat message.
- instead of storing 'provider', 'account', 'contact' in the messages table, store a 'thread_id' column instead.
  thread_id is really just the contact_id for both messages and groupchat messages.
- add getContentUriByThreadId() api, for now, retained the old getContentUriByContact(), but it's deprecated.