Skip stop if keepalive is already in stopping state

In current design, crash has been generated when stop function
has been re-entered to catch unexpected behavior. However,
it is possible to re-enter stop function if the network
disconnection occurs after stopping.

Thus, skip stop if keepalive is already in stopping state.

Test: atest ConnectivityServiceTest#testNattSocketKeepalives \
      --rerun-until-failure 60000
Bug: 167332570
Change-Id: Ic7068ad3dc990e957c37b8d87d48ebb6469b101f

tests/net/java/com/android/server/ConnectivityServiceTest.java

@@ -4292,6 +4292,32 @@ public class ConnectivityServiceTest {
         myNet = connectKeepaliveNetwork(lp);
         mWiFiNetworkAgent.setStartKeepaliveEvent(SocketKeepalive.SUCCESS);
 
+        // Check that a stop followed by network disconnects does not result in crash.
+        try (SocketKeepalive ka = mCm.createSocketKeepalive(
+                myNet, testSocket, myIPv4, dstIPv4, executor, callback)) {
+            ka.start(validKaInterval);
+            callback.expectStarted();
+            // Delay the response of keepalive events in networkAgent long enough to make sure
+            // the follow-up network disconnection will be processed first.
+            mWiFiNetworkAgent.setKeepaliveResponseDelay(3 * TIMEOUT_MS);
+            ka.stop();
+
+            // Make sure the stop has been processed. Wait for executor idle is needed to prevent
+            // flaky since the actual stop call to the service is delegated to executor thread.
+            waitForIdleSerialExecutor(executor, TIMEOUT_MS);
+            waitForIdle();
+
+            mWiFiNetworkAgent.disconnect();
+            mWiFiNetworkAgent.expectDisconnected();
+            callback.expectStopped();
+            callback.assertNoCallback();
+        }
+
+        // Reconnect.
+        waitForIdle();
+        myNet = connectKeepaliveNetwork(lp);
+        mWiFiNetworkAgent.setStartKeepaliveEvent(SocketKeepalive.SUCCESS);
+
         // Check that keepalive slots start from 1 and increment. The first one gets slot 1.
         mWiFiNetworkAgent.setExpectedKeepaliveSlot(1);
         int srcPort2 = 0;