Mark integration_ui flaky (#17319)

This test fails consistently on mac2 and mac3 with the attached Moto G4
devices but passes consistently on other machines.

Adding a delay of 1s right after driver.connect() in setUpAll() causes
it to pass on the machines in question, which suggests a race condition.
Specifically it looks like connect returns the moment Flutter Driver
identifies that the isolate is up and running, but empirically it looks
like we start running the first test before the UI is actually up. This
triggers a failure wherein we start looking for elements before they're
onstage.

Link to viewport.dart:213 at HEAD:
https://github.com/flutter/flutter/blob/b2b46659262c66ff13abc2b8016a94a47646eaad/packages/flutter/lib/src/widgets/viewport.dart#L213

Stack trace:
FlutterDriver waitFor should find text "present"

```
  DriverError: Error in Flutter application: Uncaught extension error while executing waitFor: NoSuchMethodError: The getter 'visible' was called on null.
  Receiver: null
  Tried calling: visible
  #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:46:5)
  #1      _ViewportElement.debugVisitOnstageChildren. (package:flutter/src/widgets/viewport.dart:213:36)
  #2      WhereIterator.moveNext (dart:_internal/iterable.dart:439:11)
  #3      Iterable.forEach (dart:core/iterable.dart)
  #4      _ViewportElement.debugVisitOnstageChildren (package:flutter/src/widgets/viewport.dart:214:8)
  #5      _DepthFirstChildIterator._reverseChildrenOf (package:flutter_test/src/all_elements.dart:54:15)
  #6      _DepthFirstChildIterator.moveNext (package:flutter_test/src/all_elements.dart:45:19)
  #7      CachingIterable._fillNext (package:flutter/src/foundation/basic_types.dart:252:27)
  #8      _LazyListIterator.moveNext (package:flutter/src/foundation/basic_types.dart:279:21)
  #9      WhereIterator.moveNext (dart:_internal/iterable.dart:438:22)
  #10     CachingIterable._fillNext (package:flutter/src/foundation/basic_types.dart:252:27)
  #11     _LazyListIterator.moveNext (package:flutter/src/foundation/basic_types.dart:279:21)
  #12     Iterable.isEmpty (dart:core/iterable.dart:449:33)
  #13     Iterable.isNotEmpty (dart:core/iterable.dart:456:27)
  #14     FlutterDriverExtension._waitForElement. (package:flutter_driver/src/extension/extension.dart:215:51)
  #15     FlutterDriverExtension._waitUntilFrame (package:flutter_driver/src/extension/extension.dart:197:19)
  #16     FlutterDriverExtension._waitForElement (package:flutter_driver/src/extension/extension.dart:215:11)

  #17     FlutterDriverExtension._waitFor (package:flutter_driver/src/extension/extension.dart:286:11)

  #18     FlutterDriverExtension.call (package:flutter_driver/src/extension/extension.dart:168:51)

  #19     BindingBase.registerServiceExtension. (package:flutter/src/foundation/binding.dart:370:32)
```

Removes a previous hack that no longer appears to help (adding a 1
second delay in setUpAll() does seem to work around this issue though).

dev/devicelab/manifest.yaml

@@ -193,6 +193,7 @@ tasks:
       Runs end-to-end Flutter tests on Android.
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
+    flaky: true
 
   commands_test:
     description: >

dev/integration_tests/ui/lib/driver.dart

@@ -7,10 +7,7 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_driver/driver_extension.dart';
 
 void main() {
-  enableFlutterDriverExtension(handler: (String message) async {
-    // TODO(cbernaschina) remove when test flakiness is resolved
-    return 'driver';
-  });
+  enableFlutterDriverExtension();
   runApp(new DriverTestApp());
 }