Clamp non-monotonic stats instead of dropping.

When encountering non-monotonic stats rows, recover remaining data by
clamping to 0.  In particular, this avoids edge-case where persisting
threshold checks would never trigger.  Also recover when tethering
snapshots are missing.

Bug: 5600785, 5433871, 5600678
Change-Id: I1871954ce3955cc4ac8846f9841bae0066176ffe