Use prefer for source vs prebuilts selection of widevine

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

libwvdrmengine/apex/prebuilt/Android.bp

@@ -25,27 +25,39 @@ package {
     default_applicable_licenses: ["vendor_widevine_license"],
 }
 
-apex_set {
+soong_config_module_type {
+    name: "widevine_apex_set",
+    module_type: "apex_set",
+    config_namespace: "widevine",
+    bool_variables: ["source_build"],
+    properties: ["prefer"],
+}
+
+widevine_apex_set {
     name: "com.google.android.widevine.nonupdatable",
     apex_name: "com.google.android.widevine",
     owner: "google",
     set: "com.google.android.widevine.nonupdatable.apks",
     vendor: true,
-    use_source_config_var: {
-        config_namespace: "widevine",
-        var_name: "source_build",
+    prefer: true,
+    soong_config_variables: {
+        source_build: {
+            prefer: false,
+        },
     },
 }
 
-apex_set {
+widevine_apex_set {
     name: "com.google.android.widevine.lazy",
     apex_name: "com.google.android.widevine.lazy",
     owner: "google",
     set: "com.google.android.widevine.lazy.apks",
     vendor: true,
-    use_source_config_var: {
-        config_namespace: "widevine",
-        var_name: "source_build",
+    prefer: true,
+    soong_config_variables: {
+        source_build: {
+            prefer: false,
+        },
     },
 }