Tracks isolated uid's and their parent uid.

We push events from BatteryStatsImpl if an isolated uid is added or
removed and we have a custom rule in statsd to use these events to
update our uid map. In the future, we need to use this map to
convert all incoming uid's to their host uid.

Test: Added unit-test to UidMap_test.
Change-Id: I33c0451eb2c886161f22dd12e479d216fad0940d