Add new API to scan a given channel set

Create new (hidden) APIs to scan a specified channel set. This is mainly for
location service for now, as doing frequent full scans (including DFS channel)
kills battery.

This set of new APIs contains:
- API to query available channel list for scanning, and DFS channels are marked
- API to scan a specified channel set

Selective scan results are returned in the same way as normal full scan ones.
In order not to intrude full scan results, BSS_FLUSH is NOT performed before a
selective scan. Therefore, selective scan results may contain results that's not
in the given channel set, and post-filtering may be required for location
service use.

Bug: 12610436
Change-Id: Icced18dd9b54f6dd92c0c2874d31518d56af8163