Memory leak due to no stop for IpNeighborMonitor.

1. When Wi-Fi enabled, ap0 interface is added and IpNeighborMonitor's
   start() is invoked in IpServer's constructor.
2. There's no stop for IpNeighborMonitor when IpServer stop.
3. During overnight test for Wi-Fi, networkstack is too slow to
   process request due to memory leak in IpNeighborMonitor and
   binder buffer is blocked causing exception.
Solution: Invoke IpNeighborMonitor.stop() in UnavailableState.enter().

Bug: 159097215
Test: automatic, turn on and off Wi-Fi every 6 seconds overnight
Change-Id: I8f60c13706f05306e8f25a15f7861d7ecabbc10e

Tethering/src/android/net/ip/IpServer.java

@@ -1321,6 +1321,7 @@ public class IpServer extends StateMachine {
     class UnavailableState extends State {
         @Override
         public void enter() {
+            mIpNeighborMonitor.stop();
             mLastError = TetheringManager.TETHER_ERROR_NO_ERROR;
             sendInterfaceState(STATE_UNAVAILABLE);
         }