softap: introduce new property to allow set/get of softap properties.

Softap operations need to have persistent property flags. This
commit adds softap_prop which would allow access/set/get to
persist.vendor.wifi.softap.* properties.

This is primarily used for vendor features flag such as STA+SAP
and SAP+SAP.

CRs-Fixed: 2217480
Change-Id: I21e1af448612f4a64512ae5c2b7d704c05d88454