Add KeyValuePairs support to StatsEvent.

- Expose atom id in StatsEvent.
- Sync constants with native stats_event.

Bug: 143079343
Bug: 143286399
Test: m -j
Change-Id: Icaa2d90b40d5da7571444182ead03b54b7b25867
Merged-In: Icaa2d90b40d5da7571444182ead03b54b7b25867