Add HotwordMetricsLogger for statistics logging.

This change only provide a utilility class to help hotword dection
metrics logging. The log writing will on the follow up changes.

Android Metrics Design Review : eldar/276723226

Bug: 207717787
Test: build pass can boot success
Change-Id: I3f9f597554aa9e65a20076cc1b58072c8e390e30
Merged-in: If436aae1b4ebdb06c7bd5a1c0b8d6df4e94488cd
(cherry picked from commit e13c17d233e74265fa68d7470a111069d16ea50b)