Gracefully handle integer overflows.

Avoid recording negative data by doing integer-based
math as much as possible, but switch to double-based
math if we detect that we'd end up causing an overflow.

Test :

    - Builds, Boots
    - High data throughput scenarios
    - NetworkStatsHistoryTest, NetworkStatsCollectionTest,
      NetworkStatsTest

Bug: 119527458
Change-Id: I55d4aa649bea3ae201ad513074ff2b71cb918dad