Add wifi_module/source_build Soong config variable

As an optional module, wifi has its own Soong config variable. This
change ensures that it is controlled by the same environment variable
as the rest of the modules.

Bug: 233965247
Test: # Unpack wifi snapshot that uses wifi specific Soong config.
      m nothing
      # Without this the build fails as it uses the wifi prebuilts
      # which do not work in AOSP.
      # With this change the build passes as it uses the wifi
      # sources.
Change-Id: Iea0ad09182a162b8f5ddf1403f65a3bbffb9ef08