Also add minidns-core as dependency to smack-core. This requires
increasing the minimum required Android SDK level to 9, as this is
what MiniDNS requires.
And replace all instances where String.Builder.append() is called with a
String of length one with append(char).
Also adds StringUtils.toStringBuilder(Collection, String).
We now treat warnings as errors (-Werror).
In order to do so, it was necessary to remove Java7HostnameVerifier
since it depended on internal properietary API. XmppHostnameVerifier
take the place of it.