WifiManager/Scanner: Expose @hide APIs as @SystemApi

Expose @hide APIs in WifiManager/WifiScanner as
@SystemApi so that they can be referenced by external
callers (mostly Settings).

Bug: 143970861
Test: atest FrameworksWifiApiTests
Change-Id: I956290e2a3aca3f4946da52d7e271082824dd06c