Statsd always includes snapshot of uid map.

Statsd will contain at least one snapshot of the uid map. The
previous design was not very robust in case a snapshot was missing.

Also fixes subtle bug with updating the isolated uid mapping since
this should always be kept up to date even if there are no metrics
being used (since metrics may be added later after the isolated uid
was created).

Test: Checked that unit-tests pass on marlin-eng.
Change-Id: I99754ed9016d980564e409b0946a46b398fd12b7