wifi: Split out initialize and start in WifiLegacyHal

We need to separate these because we need to invoke start() after every
firmware mode change (chip reconfigure).

While there,
1. Make InterfaceTool a member of the class.
2. Make the stop() symmetric with start(). i.e interface is set
down on stop immediately instead of waiting for the thread to stop.

Bug: 31997422
Test: Compiles
Change-Id: I202afcc70571188dc076a841249761bc97fcf817