Adds guardrail for memory usage for statsd uid map.

Checks if current memory usage of uid map is above a configured limit
and if so, we start deleting snapshots. If there are no more
snapshots, we begin deleting two of the deltas. Also records stats
in the guardrail StatsdStats. Also fixes an edge case where a config
is added after the snapshots are added. We request a snapshot of all
installed uid's at that moment. Finally, adds the uid map memory size
when determining if we should send a broadcast to trigger collection.

Test: Added unit-tests and check they pass on marlin.
Change-Id: Id5d86378bd1efe12a06b409164c777c0c6f4e3ab