Fix canceling renew alarms.

1. Entering DhcpBoundState cancels the renew alarm, but at that
   point the renew alarm is guaranteed not to have been scheduled.
   This is harmless, but results in an "unknown listener" message
   in the AlarmManager logs.
2. We don't cancel the renew alarm when exiting DhcpBoundState.
   This is also harmless, because that alarm does nothing except
   in DhcpBoundState, and we cancel it whenever we enter
   DhcpBoundState. But canceling it on exit is more correct.

Change-Id: I60dfcf00f243253b81b8906540e0a6218a7a489c