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/tools/create_api_docs.dart

@@ -1265,7 +1265,7 @@ class FlutterInformation {
     info['engineRevision'] = flutterVersion['engineRevision'] as String;
     final File engineRealm = flutterRoot
         .childDirectory('bin')
-        .childDirectory('internal')
+        .childDirectory('cache')
         .childFile('engine.realm');
     info['engineRealm'] = engineRealm.existsSync() ? engineRealm.readAsStringSync().trim() : '';