Wait for IpClient to shutdown

This makes sure that we never accidentally have two IpClients vying
for control of the same interface.

Test: as follows
    - built
    - flashed
    - booted
Bug: 62476366
Change-Id: Iab4ca7c2445e70b6f8beaf5dc6921cb0c6cd56ed
diff --git a/java/com/android/server/ethernet/EthernetNetworkFactory.java b/java/com/android/server/ethernet/EthernetNetworkFactory.java
index d464879..f95500e 100644
--- a/java/com/android/server/ethernet/EthernetNetworkFactory.java
+++ b/java/com/android/server/ethernet/EthernetNetworkFactory.java
@@ -355,8 +355,10 @@
         void stop() {
             if (mIpClient != null) {
                 mIpClient.shutdown();
+                mIpClient.awaitShutdown();
                 mIpClient = null;
             }
+
             // ConnectivityService will only forget our NetworkAgent if we send it a NetworkInfo object
             // with a state of DISCONNECTED or SUSPENDED. So we can't simply clear our NetworkInfo here:
             // that sets the state to IDLE, and ConnectivityService will still think we're connected.