Prepares wpa_supplicant for use by a wifi HAL vendor APEX.

- Allows wpa_supplicant HIDL to read from an APEX for config files.
- Creates a cc_defaults with soong config vars for wpa_supplicant that
  customized implementations can override.

Bug: 201817549
Test: Build a wifi hal vendor APEX on Cuttlefish.
Test: Connect to wifi; atest CtsWifiTestCases
Change-Id: I92f98bb61a67724674374279f6c7a8233a7b8559
3 files changed