build: enable bundle_dts for core package (#28884)

`ng_module` will now include an `src/r3_symbol.d.ts` when compiling the core package under `ngc` togather with `dts bundling`, This is due that `ngcc` relies on this file to be present, but the `r3_symbols` file which is not part of our public api.

With this change, we can now ship an addition dts file which is flattened.

PR Close #28884

packages/core/testing/BUILD.bazel

@@ -9,8 +9,6 @@ ng_module(
     srcs = glob(
         ["**/*.ts"],
     ),
-    # PR to support this https://github.com/angular/angular/pull/28884
-    bundle_dts = False,
     deps = [
         "//packages:types",
         "//packages/compiler",