RootShell: Add setRetryExitCode() feature

Add RootShell support for retrying commands that can experience transient
failures.
Api: Retry "iptables" commands on exit status 4

Various users have reported intermittent problems in which iptables
returns exit status 4 (iptables: Try again).  This can happen if the
Android system is updating the "filter" table (any chain) at the same time
we are.  These updates generally aren't modifying the same chain or rules,
so they shouldn't conflict with each other in any other way.

The problem can be reproduced by applying the AFWall rules while running
this loop in another root shell:

    iptables -N foobar
    while :; do iptables -A foobar -d 1.2.3.4 -j ACCEPT ; \
        iptables -D foobar -d 1.2.3.4 -j ACCEPT ; done

We will retry these transactions up to 10 times.
Remove a couple of unused imports
Merge pull request #192 from cernekee/exit4-v1

Work around intermittent iptables exit status 4 errors