Statsd: pull once per event time

If a pull happens at the same event time, we should reuse the existing
data, regardless of whether or not the cool down has been met. For
example, if an app upgrade happens at time t, and two metrics need to
pull atom a, if metric one pulls at time t, but metric two initiates the
pull at time t+2, we should still reuse the pull from time t since that
is when the app upgrade happened.

Bug: 156294650
Test: atest statsd_test

Change-Id: I4efc49545093f6683bf6dd89ed68c5dfa5b44d8f