Refactor Interval struct

Create another struct that contains the vector of Intervals.
This new struct will later contain a ConditionTimer to support slicing
condition_true_nanos by state.

Bug: 165817662
Test: `m statsd`
Test: `m statsd_test`
Test: `m`
Test: `atest statsd_test`
Change-Id: Id1509ac04deee5719aaf2dc4073df62516a1edf2