Puller API: Unit tests.

Tests 4 key things: A successful pull, a failed pull and two timeout
cases.

Timeout case 1: StatsPullerCallback should stop early and return true.
Timeout case 2: StatsCallback should note that the pull timed out,
notify statsd stats, and return false.

Test: bit statsd_test:*
Bug: 145310627
Change-Id: Id87089f04e1cf54a622b3f15585341ecdcd21f7f