WifiScanner: Add scan request type to ScanSettings

This new 'type' parameter in ScanSettings is used to convey the goal of
the scan to the underlying wifi chip. This parameter is intentionally
abstract to let the wifi chip vendors decide what type of scan to
perform on devices which support multiple scan modes (DBS, MCC, etc).

Bug: 68335251
Test: Unit tests
Change-Id: Iabddad980128848938b6e2223ad199345812615b