Fix and cleanup smart replies checking

1. We now apply smart replies to the first action button that has
   (freeform) remote input. For example, if the notification have
   both reply and reply all buttons, smart replies will be applied
   to the first button only.
2. Enforced getAllowGeneratedReplies check in system generated
   smart replies.
3. Fixed an bug that smartRepliesAdded is not called for system
   generated smart replies.

BUG: 111546109
BUG: 111406942

Test: atest com.android.server.notification.NotificationTest
Test: Check apps generated smart replies via Messenger
Test: Check system generated smart replies via Hangouts

Change-Id: I4db34f557f7e9988be612e4162347b86393d1faf