[iOS] Fix:Keyboard inset is not correct when presenting and native ViewController on FlutterViewController (flutter#29862)

Author: luckysmg

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -1193,9 +1193,10 @@ - (void)startKeyBoardAnimation:(NSTimeInterval)duration {
       }
       completion:^(BOOL finished) {
         if (self.displayLink == currentDisplayLink) {
+          // Indicates the displaylink captured by this block is the original one,which also
+          // indicates the animation has not been interrupted from its beginning. Moreover,indicates
+          // the animation is over and there is no more animation about to exectute.
           [self invalidateDisplayLink];
-        }
-        if (finished) {
           [self removeKeyboardAnimationView];
           [self ensureViewportMetricsIsCorrect];
         }

shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

@@ -173,9 +173,17 @@ - (void)testkeyboardWillChangeFrameWillStartKeyboardAnimation {
                     @"UIKeyboardAnimationDurationUserInfoKey" : [NSNumber numberWithDouble:0.25],
                     @"UIKeyboardIsLocalUserInfoKey" : [NSNumber numberWithBool:isLocal]
                   }];
+
+  XCTestExpectation* expectation = [self expectationWithDescription:@"update viewport"];
+  OCMStub([mockEngine updateViewportMetrics:flutter::ViewportMetrics()])
+      .ignoringNonObjectArgs()
+      .andDo(^(NSInvocation* invocation) {
+        [expectation fulfill];
+      });
   id viewControllerMock = OCMPartialMock(viewController);
   [viewControllerMock keyboardWillChangeFrame:notification];
   OCMVerify([viewControllerMock startKeyBoardAnimation:0.25]);
+  [self waitForExpectationsWithTimeout:5.0 handler:nil];
 }
 
 - (void)testEnsureViewportMetricsWillInvokeAndDisplayLinkWillInvalidateInViewDidDisappear {