WifiScanner: Add PNO public API

Add the public interface for starting/stopping PNO scan.

This is the internal flow that is going to be used for PNO scan:
1. Set Pno list
2. Start background scan

This flow will not work as is for supplicant, since background scan is
done manually today. So, we'll need a flag to indicate that this
background scan is targetted for PNO.

BUG: 27167559
Change-Id: I3f6921c5d03f7d9e8ad1847e57e12fb38df6c9c9
TEST: Compiles & unit-test passes