Updates uidmap to update snapshots and upload.

We send a snapshot of all installed apps with their uids every time
a user is added or removed and when statsd is started.
We keep track of the latest timestamp when a config key has retrieved
the UID map data. This allows us to remove older data when we're
guaranteed that all config sources have retrieved the old data.

Test: Added more unit tests to UidMap_test and passed on marlin-eng.
Change-Id: I34a3d61e75eedec44b98d896d7f6db0bc383f46a