NetworkStats: more robust subtraction for deltas

This patch ensures that subtract() between two NetworkStats object will
return a delta with no negative entries in all cases.

When the stats delta contains some negative values, there are clamped to
0. Some logging is added when this happens.

This is what's expected by NetworkStatsHistory#recordData().

Bug: 64365917
Bug: 65439160
Test: runtest frameworks-net
Merged-In: I16e97e73f600225f80e0ce517e80c07c6f399196
Merged-In: I2ac0bc3914cb65ae8ee27921856d698dc59624b2
Merged-In: I67d5dc4b52b254748ff17fe1e16c2eeb1d03c30d
Merged-In: Ib488fb034f72c92f19916490981342a3ef2eb33b

(cherry picked from commit ad5e2827)

Change-Id: Ic86b65a65a2517c871221f8784088ec1de18f534