Define HotwordTrainingData

This structure will be egressed from the active HotwordDetectionService
when it determines useful training events. We plan to extend the hotword
detection service callback to allow the egress of HotwordTrainingData
by adding  onTrainingData(HotwordTrainingData data) method to the callback.

Change-Id: I476fe9180a5fd58e56197d606fbdac04b9792af2
Test: atest --no-bazel-mode CtsVoiceInteractionTestCases:HotwordDetectionServiceBasicTest
Bug: 291599840