Fix leak of DhcpServer on tethering stop error

Tactical, minimal fix to make sure the DhcpServer is stopped when
disabling tethering. The current code may throw if the interface is gone
when calling mNMService.setInterfaceConfig() on teardown. Make sure DHCP
is stopped regardless of what happens.

A longer term fix will be to figure out the right flow to teardown the
interface and avoid errors on setInterfaceConfig().

Bug: 124520692
Test: Manual: no more socket leaked when stopping USB tethering
Change-Id: Ib87d89a2e5fdb64b8f3dfae48d82c970f3ae6161

services/net/java/android/net/ip/IpServer.java

@@ -433,6 +433,9 @@ public class IpServer extends StateMachine {
                 }
             }
             ifcg.clearFlag("running");
+
+            // TODO: this may throw if the interface is already gone. Do proper handling and
+            // simplify the DHCP server start/stop.
             mNMService.setInterfaceConfig(mIfaceName, ifcg);
 
             if (!configureDhcp(enabled, (Inet4Address) addr, prefixLen)) {
@@ -440,6 +443,14 @@ public class IpServer extends StateMachine {
             }
         } catch (Exception e) {
             mLog.e("Error configuring interface " + e);
+            if (!enabled) {
+                try {
+                    // Calling stopDhcp several times is fine
+                    stopDhcp();
+                } catch (Exception dhcpError) {
+                    mLog.e("Error stopping DHCP", dhcpError);
+                }
+            }
             return false;
         }