wifi: Restructure wifi legacy HAL implementation

Restructured the existing code to create a new class called
|WifiLegacyHal|. This class will be used as a wrapper to invoke
all the legacy HAL functions and handle the "C" style callbacks.

Bug: 31936700
Test: mmma -j32 hardware/interfaces/wifi/1.0/default
Change-Id: I63e8543f49886f8446101320a97d1e96e30d1035