Disable zen rule preferences with invalid activities

While resolveActivity is used to determine whether an Intent can be handled by something, this doesn't catch the case of explicit intents whose activity class doesn't exist. Here we check for it through PackageManager.queryIntentActivities instead for existing zen rules (if they were added when the activity exists, but it no longer does).

For new rules, check the validity of the activity for external rules before adding them to the list.

Bug: 238144390
Test: manual via DND app
Change-Id: Ia920ca792f9c17a5d684baf877c882ce7fadffd6