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
Test: runtest frameworks-net
Change-Id: I16e97e73f600225f80e0ce517e80c07c6f399196