lowpan: Introduce new unit tests for data classes

This change introduces new unit tests which confirm that
various data classes are being property marshalled
between their Java and C++ implementations.

Bug: b/63707448 b/63708348
Change-Id: I41d590b1e77dc41873c4b9e9bf1b7f1bf859f74e
Test: These tests both compile and pass
18 files changed