ValueMetric overhaul

This is savaged from the large cl.

1. Simplify the logic in ValueMetricProducer.
1.1 for pull data on bucket boundary, we pull on bucket end, instead of
playing with timestamp twice.
1.2 for data that require diffing, we keep a rolling diff base that gets
updated.
1.3 Now we check condition in onMatchedLogEventInternalLocked for pushed atoms. For pulled atoms, check before commit time. This was very error prone in P and caused multiple bugs. It is much simpler now.
2. Treat pushed and pulled atoms the same way and share the same
aggregation types.
4. Allow decreasing values for diffing.
5. Allow diffing for pushed atoms.
6. For diff based aggregation, if the diff value is zero, we skip
output.

Bug: 117224984
Bug: 115683963
Bug: 117975256
Bug: 113268259
Test: unit test
Change-Id: I6ee306e9f6e5a166b392c443594704e7d2792ef5