widevine currenty uses `use_source_config_var` and product variables to
ensure that products gets the correct selection of source or prebuilts
of widevine apex.
`use_source_config_var` is being deprecated from Soong to unfiy the
mechanisms for source vs prebuilt selection. This CL transitions
widevine to a different mechanism `prefer` for prebuilt selection to aid
the deprecation.
Test: lunch cf_x86_64_phone-trunk_staging-userdebug; m nothing; aninja
-t path droid
vendor/widevine/libwvdrmengine/apex/prebuilt/com.google.android.widevine.nonupdatable.apks;
// no path exists, i.e. uses source
Test: lunch osprey-trunk_staging-userdebug; m nothing; aninja -t path
droid vendor/widevine/libwvdrmengine/apex/prebuilt/com.google.android.widevine.nonupdatable.apks;
// path exists; i.e. uses prebuilts
Bug: 332379718
Change-Id: I78800aee49f1de83ea2ce8160923362871806d87