Implement restore functions on Android and iOS (#6322)

This will not be active on the Framework until https://github.com/flutter/flutter/pull/22221 lands.

shell/platform/android/io/flutter/plugin/platform/PlatformPlugin.java

@@ -58,6 +58,9 @@ public class PlatformPlugin implements MethodCallHandler, ActivityLifecycleListe
             } else if (method.equals("SystemChrome.setEnabledSystemUIOverlays")) {
                 setSystemChromeEnabledSystemUIOverlays((JSONArray) arguments);
                 result.success(null);
+            } else if (method.equals("SystemChrome.restoreSystemUIOverlays")) {
+                restoreSystemChromeSystemUIOverlays();
+                result.success(null);
             } else if (method.equals("SystemChrome.setSystemUIOverlayStyle")) {
                 setSystemChromeSystemUIOverlayStyle((JSONObject) arguments);
                 result.success(null);
@@ -224,6 +227,10 @@ public class PlatformPlugin implements MethodCallHandler, ActivityLifecycleListe
         }
     }
 
+    private void restoreSystemChromeSystemUIOverlays() {
+        updateSystemUiOverlays();
+    }
+
     private void setSystemChromeSystemUIOverlayStyle(JSONObject message) {
         Window window = mActivity.getWindow();
         View view = window.getDecorView();

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

@@ -51,6 +51,9 @@ using namespace shell;
   } else if ([method isEqualToString:@"SystemChrome.setEnabledSystemUIOverlays"]) {
     [self setSystemChromeEnabledSystemUIOverlays:args];
     result(nil);
+  } else if ([method isEqualToString:@"SystemChrome.restoreSystemUIOverlays"]) {
+    [self restoreSystemChromeSystemUIOverlays];
+    result(nil);
   } else if ([method isEqualToString:@"SystemChrome.setSystemUIOverlayStyle"]) {
     [self setSystemChromeSystemUIOverlayStyle:args];
     result(nil);
@@ -116,11 +119,10 @@ using namespace shell;
 
   if (!mask)
     return;
-  [[NSNotificationCenter defaultCenter] postNotificationName:@(kOrientationUpdateNotificationName)
-                                                      object:nil
-                                                    userInfo:@{
-                                                      @(kOrientationUpdateNotificationKey) : @(mask)
-                                                    }];
+  [[NSNotificationCenter defaultCenter]
+      postNotificationName:@(kOrientationUpdateNotificationName)
+                    object:nil
+                  userInfo:@{@(kOrientationUpdateNotificationKey) : @(mask)}];
 }
 
 - (void)setSystemChromeApplicationSwitcherDescription:(NSDictionary*)object {
@@ -138,6 +140,10 @@ using namespace shell;
       ![overlays containsObject:@"SystemUiOverlay.top"];
 }
 
+- (void)restoreSystemChromeSystemUIOverlays {
+  // Nothing to do on iOS.
+}
+
 - (void)setSystemChromeSystemUIOverlayStyle:(NSDictionary*)message {
   NSString* style = message[@"statusBarBrightness"];
   if (style == (id)[NSNull null])
@@ -160,9 +166,7 @@ using namespace shell;
     [[NSNotificationCenter defaultCenter]
         postNotificationName:@(kOverlayStyleUpdateNotificationName)
                       object:nil
-                    userInfo:@{
-                      @(kOverlayStyleUpdateNotificationKey) : @(statusBarStyle)
-                    }];
+                    userInfo:@{@(kOverlayStyleUpdateNotificationKey) : @(statusBarStyle)}];
   } else {
     // Note: -[UIApplication setStatusBarStyle] is deprecated in iOS9
     // in favor of delegating to the view controller