Unify the way we process events on condition change and bucket

boundaries.

- We now always process empty data and events that exceeds the max delay
in a consistent way

- Fix one bug which caused base data not to be reset, e.g.
bucket h+1 on condition change to true, we pull the following data (key A / value 1, key B / value 2)
bucket h+2 on bucket boundary, we pull the following data (key A / value 2)
In this case the previous code, did not reset Key B. It should be reset
since it is not present in the most recent data.

Test: atest statsd_test
Bug: 124046337
Change-Id: I19456bbe1529e72befbb621be185ce24bd65077a