wlan: Add periodic scan to address Stickiness issue.

In case of 11K with roaming, when low RSSI indication
received from FW, host unable to found a candidate from
neighbor report request, occupied channel list scanning
and ini isRoamOffloadScanEnabled is disabled,
thus host does not trigger any scan and zero candidates found.
Also if low RSSI indication is received in the same region,
host do not process it.

Fix is to introduce Periodic scan, when host does not get
candidate from neighbor report request and occupied
channel list scan in case of 11K with roaming.
When host gets candidate with better RSSI, periodic scan is stop.

Change-Id: Ie9f875b0ce742e3a8d4dab7c696da6972d2fe374
CRs-Fixed: 3152359