Convert the existing AIDL binder interface of wpa_supplicant to HIDL.
Summary of changes:
1. Every HIDL method returns an instance of |SupplicantStatus|
along with any others params (for getters). This is needed to return the
equivalent of |Binder::Status| to indicate errors in the args passed,
stale proxy, etc.
2. All constants are changed to enums. There were some constants which
should have been enums in the first place, but wasn't because AIDL did
not support enums. But, there are others which should be standalone
constants, but are enums now because constants are not supported in HIDL.
3. Conform to HIDL style guide.
Bug: 31365276
Test: `mmm -j32 hardware/interfaces/wifi/1.0/`
Change-Id: Iba753e279fd260788d8628ea2f5c2281b5844095
