Pivot network statistics to use DataInput/Output.

Using these generalized interfaces is more flexible, since it enables
us to pivot the implementation being used internally.  In particular,
an upcoming CL will pivot them to use a more efficient alternative.

This is a no-op refactoring.

Bug: 176777285
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: Ibd4717174cf1f136e9d5d80172ecb6e493265306