In NetworkValidator, make sure timeout happens for available but

unvalidated network.

We used to rely on ConnectivityManager's API to do timeout. But problem
is, if a network becomes available but never validated, there's no
timeout and we fail to send any result. So now we are doing timeout
ourselves.

Bug: 134526729
Test: manual and unittests
Change-Id: If47def80a7176b1a45a0490561b473e16d19d945