[WIFICOND] Formalize the wificond AIDL interface

The wificond daemon provides an interface for the framework to interact
with the operating system when managing Wi-Fi. Since wificond is outside
the mainline module, the AIDL interface has to be formalized as an API
surface.

Bug: 140062898
Test: atest android.net.wifi
Test: atest com.android.server.wifi.aware
Test: (CTS) atest android.net.wifi.cts
Test: scan, association, SoftAP operations manually tested, kill wificond
Change-Id: Iee0fef6a454fdd84b1d0c59516d4746ddc2a4ce5