Dump last 24 hours of UsageStats events

On calling dumpsys usagestats, the last 24 hours of events will be
dumped rather than the events since the arbitrary time point
(also increase timestamp granularity to seconds)

Fixes: 73657997
Test: manual (adb shell dumpsys usagestats)

Change-Id: I92f1f17b4119d2023ed9b81e0610c19f62503bf0