Move NetworkStackConstants to libs/net

The class only contains constants that should be shareable between
different modules and tests.

Bug: 168868607
Test: m
Merged-In: Ia69fed211ac9485beabc280cd17af2826b8ea7f9
Change-Id: Ia69fed211ac9485beabc280cd17af2826b8ea7f9
17 files changed