Remove support for filtering tether stats.

The framework does not use this filtering, so it's just
dead code. It also requires that NatController publicly
expose its list of interface pairs.

Also make the parsing code a bit stricter - for example, return
an error if any of the lines (except the headers) fail to parse.

Bug: 32163131
Bug: 64995262
Test: bullhead builds and boots
Test: netd_{unit,integration}_test pass
Test: output of "adb shell ndc bandwidth gettetherstats" looks correct
Change-Id: Ib7440f935809c59d8b48396764cc63eb95f509b4