Gracefully handle integer overflows.

Try sticking with integer-based math as much as possible for speed,
but switch to double-based math if we detect that we'd end up
causing an overflow.

New tests to verify.

Test: bit FrameworksNetTests:com.android.server.net.NetworkStatsCollectionTest
Bug: 65257769
Change-Id: I1ae35599be134f81850c0a3d86928b057fba1eff