Improve listenForPrivateChats

- use anonymous class instead of inner class
- decrease indentation level

Also removed comment about ToContainsFilter. Such a filter never made
sense, see also RFC 6120 8.1.1.1:

"It is the server's responsibility to deliver only stanzas that are
addressed to the client's full JID or the user's bare JID; thus, there
is no need for the client to check the 'to' address of incoming
stanzas."