Add missing nullness annotations to Notification.Action and its Builder.

It is documented that Notification.Action.actionIntent can be null and
what that results
(https://developer.android.com/reference/android/app/Notification.Action#actionIntent), but annotations and equivalent documentations are
missing from the builder and the Action's constructor.

Test: N/A, no functional API change.

Change-Id: Ie6593057e17cb485bdc69988ce2eba10fc3fcfd8