SOFTAP: implement SOFTAP new requirement in framework

If framework recevied the STA (dis)connect message from netd,
it will try to read device info from dnsmasq and broadcast
an state change event via intent.
Meanwhile, framework will maintain the conneted Sta list which
can be access from upper layer.

Change-Id: I834f307b285d7227c4f651f74753ecdd1cd262e3
CRs-Fixed: 685913