Make tethering ignore bad inputs

Tethering was re-evaluating everything on any connectivity change.
This was causing problems when mobile data was not available as each
re-eval was again requesting a connection which would fail and loop.
Ignore FAILED states to break this cycle in a minimal change.

bug:6668233
Change-Id: Ia1dfc6abff1fe464e9501b50185cdb8d0fbb7ff6