Start using bin/cache/engine.{stamp|realm} instead of bin/internal/engine.{realm|version}. (#164352)

Towards https://github.com/flutter/flutter/issues/164315.

See also:
https://github.com/flutter/flutter/blob/master/docs/tool/Engine-artifacts.md.

There are more usages in `flutter/flutter`, but some will require more
specialized review (i.e. from release folks, or the Dart SDK team), so
I'll split those off.

~~Requires https://github.com/flutter/flutter/pull/164317 to merge
first.~~ ✅

dev/devicelab/bin/tasks/engine_dependency_proxy_test.dart

@@ -59,7 +59,7 @@ tasks.register("printEngineMavenUrl") {
 
         String realm =
             File(
-              path.join(flutterDirectory.path, 'bin', 'internal', 'engine.realm'),
+              path.join(flutterDirectory.path, 'bin', 'cache', 'engine.realm'),
             ).readAsStringSync().trim();
         if (realm.isNotEmpty) {
           realm = '$realm/';