Use QStringList::join(QChar) overload where applicable [QtDBus]

This is an automated change performing the following replacements:
                    join\("(.)"\) -> join('\1')
  join\(QLatin1String\("(.)"\)\)  -> join(QLatin1Char('\1'))
  join\(QStringLiteral\("(.)"\)\) -> join(QLatin1Char('\1'))

Change-Id: I807b0e88ac71a0cb367fb4170cca8f2cb0ad43f3
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>