storaged: add --hours flag for dumpsys

1. Add a flag to report IO usage for last N hours.
2. Change interval back to 1 hour so that we have finer usage data.
3. Don't clear events after dumpsys call. Use a max buffer limit.
4. Clear old events if they're more than 5 days old or when no room
for new events.
5. Skip uids with no IO usage to save space.
6. Replace interval with a timestamp in event entry.

Test: adb shell dumpsys storaged --hours 2
Bug: 34198239
Bug: 33086174
Change-Id: I66e8fb6ec155584115fab817c3ed2c78e637ac40