Add unit test for shared/unshared interface quotas

Test: as follows
    - built
    - flashed
    - booted
    - "runtest -x .../netd_unit_test.cpp" passes
Bug: 28362720
Bug: 38143143

Change-Id: I0b962898f9e3d7e86d5c0d0d01b79b3e3543b5ee
Merged-In: Ia3f30bffd29255481304dfaf8ce16365d7df6261