Improve iptables timeout behaviour.

1. Increase the default timeout from 1s to 5s. This is necessary
   for as long as our version of iptables sleeps for 1 second at
   a time while the iptables lock is contended.
2. When a timeout occurs, kill the process to ensure that if it
   recovers, any output is not returned to subsequent commands.

Add corresponding unit tests.

While I'm at it:

- Ensure that iptables commands that take an output string clear
  the output string before appending to it. Otherwise, callers
  that passed the same output string object to two separate
  iptables commands would think the second command returned both
  outputs. This does not affect any existing callers.
- Delete some unused code.

Bug: 35634318
Test: netd_{unit,integration}_test pass
Change-Id: Ife3dfd328ea82f2e93fb903fcf3660a13078b7b5