Nullability fixes for HotwordDetectionService APIs.

* Make the Hotword*Result payloads NonNull. HotwordDetectedResult on the
  VoiceInteraction side is still @Nullable since the EventPayload is
  used for other result types too.
* Remove @NonNull for a primitive return type.

Bug: 184962614
Test: atest CtsVoiceInteractionTestCases
Change-Id: Ib57368dab442f80805233864dc8fd86249f677cf