fix(cdk-experimental/listbox): clean up the listbox API and make it work with forms (angular#24920)

* fix(cdk-experimental/listbox): clean up the listbox API and make it work with forms

* fix(cdk-experimental/listbox): address feedback

* fix(cdk-experimental/listbox): add forms validator support

* fix(cdk-experimental/listbox): address feedback

* fix(cdk-experimental/listbox): address feedback

* fix(cdk-experimental/listbox): address feedback

* fix(cdk-experimental/listbox): fix tests

* fix(cdk-experimental/listbox): fix lint issues

* fix(cdk-experimental/listbox): fix tests

* fix(cdk-experimental/listbox): address feedback

Author: mmalerba

src/cdk-experimental/listbox/BUILD.bazel

@@ -28,6 +28,7 @@ ng_test_library(
         "//src/cdk-experimental/combobox",
         "//src/cdk/keycodes",
         "//src/cdk/testing/private",
+        "@npm//@angular/common",
         "@npm//@angular/forms",
         "@npm//@angular/platform-browser",
     ],