Improve parsing. Add method for de-serializing Strings

Move some duplicate code from XMPP(TCP|BOSH)Connection to
PacketParserUtils. Remove TestUtils as the method now part of Smack's
public API in PacketParserUtils.
Remove schedule() from XMPPConnection interface

The idea that we abstract the scheduling of tasks on Android over this
method turned out to unnecessary. schedule() was also not really part of
the *public* XMPPConnection API, so it's good that it's gone.