turned on background isolate platform channel tests for android (#111279)

dev/integration_tests/channels/android/app/src/main/java/com/yourcompany/channels/MainActivity.java

@@ -29,6 +29,15 @@ public class MainActivity extends FlutterActivity {
     setupMessageHandshake(new BasicMessageChannel<>(dartExecutor, "std-msg", ExtendedStandardMessageCodec.INSTANCE));
     setupMethodHandshake(new MethodChannel(dartExecutor, "json-method", JSONMethodCodec.INSTANCE));
     setupMethodHandshake(new MethodChannel(dartExecutor, "std-method", new StandardMethodCodec(ExtendedStandardMessageCodec.INSTANCE)));
+
+    BasicMessageChannel echoChannel =
+        new BasicMessageChannel(dartExecutor, "std-echo", ExtendedStandardMessageCodec.INSTANCE);
+    echoChannel.setMessageHandler(new BasicMessageChannel.MessageHandler(){
+      @Override
+      public void onMessage(final Object message, final BasicMessageChannel.Reply reply) {
+        reply.reply(message);
+      }
+    });
   }
 
   private <T> void setupMessageHandshake(final BasicMessageChannel<T> channel) {

dev/integration_tests/channels/lib/main.dart

@@ -173,7 +173,7 @@ class _TestAppState extends State<TestApp> {
     () => basicStringMessageToUnknownChannel(),
     () => basicJsonMessageToUnknownChannel(),
     () => basicStandardMessageToUnknownChannel(),
-    if (Platform.isIOS)
+    if (Platform.isIOS || Platform.isAndroid)
       () => basicBackgroundStandardEcho(123),
   ];
   Future<TestStepResult>? _result;