Copy system version of the api-versions.xml file to dist

Copies the system version of the `api-versions.xml` file to a file
`api-versions_system.xml` in the top level dist directory.

Bug: 190665366
Test: m sdk dist
      # Check to make sure that the file exists.
Change-Id: Ie139f81a819eb5f389363ffdefa7bc42d36804e0

api/StubLibraries.bp

@@ -900,6 +900,15 @@ droidstubs {
     ],
     api_levels_sdk_type: "system",
     extensions_info_file: ":sdk-extensions-info",
+    dists: [
+        // Make the api-versions.xml file for the system API available in the
+        // sdk build target.
+        {
+            targets: ["sdk"],
+            dest: "api-versions_system.xml",
+            tag: ".api_versions.xml",
+        },
+    ],
 }
 
 // This module can be built with: