x11client: Replace activityList with m_activityList
KWin does not update activity status when setting _KDE_NET_WM_ACTIVITIES
to nullUuid outside of KWin, which causes "All Activities" not working
as expected for KWindowSystem.
X11Client::activityList is not kept in sync with
AbstractClient::m_activityList. Move m_activityList from private to
protected, and use it in X11Client::readActivities.
Thank @vladz for pointing out the right fix.