Migrate testing_app to UIScene lifecycle (#2836)

## Summary
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- Migrate `testing_app` iOS project to the UIScene lifecycle as per the
[migration
guide](https://docs.flutter.dev/release/breaking-changes/uiscenedelegate)
- Update `AppDelegate.swift` to use `@main`,
`FlutterImplicitEngineDelegate`, and move plugin registration to
`didInitializeImplicitFlutterEngine`
- Add `UIApplicationSceneManifest` to `Info.plist`

Part of flutter/flutter#176957

## Test plan
- [x] Ran `testing_app` on iOS simulator — app launches and works
correctly

---------

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

testing_app/ios/Runner/Info.plist

@@ -2,6 +2,8 @@
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
+	<key>CADisableMinimumFrameDurationOnPhone</key>
+	<true/>
 	<key>CFBundleDevelopmentRegion</key>
 	<string>$(DEVELOPMENT_LANGUAGE)</string>
 	<key>CFBundleDisplayName</key>
@@ -24,6 +26,29 @@
 	<string>$(FLUTTER_BUILD_NUMBER)</string>
 	<key>LSRequiresIPhoneOS</key>
 	<true/>
+	<key>UIApplicationSceneManifest</key>
+	<dict>
+		<key>UIApplicationSupportsMultipleScenes</key>
+		<false/>
+		<key>UISceneConfigurations</key>
+		<dict>
+			<key>UIWindowSceneSessionRoleApplication</key>
+			<array>
+				<dict>
+					<key>UISceneClassName</key>
+					<string>UIWindowScene</string>
+					<key>UISceneConfigurationName</key>
+					<string>flutter</string>
+					<key>UISceneDelegateClassName</key>
+					<string>FlutterSceneDelegate</string>
+					<key>UISceneStoryboardFile</key>
+					<string>Main</string>
+				</dict>
+			</array>
+		</dict>
+	</dict>
+	<key>UIApplicationSupportsIndirectInputEvents</key>
+	<true/>
 	<key>UILaunchStoryboardName</key>
 	<string>LaunchScreen</string>
 	<key>UIMainStoryboardFile</key>
@@ -43,9 +68,5 @@
 	</array>
 	<key>UIViewControllerBasedStatusBarAppearance</key>
 	<false/>
-	<key>CADisableMinimumFrameDurationOnPhone</key>
-	<true/>
-	<key>UIApplicationSupportsIndirectInputEvents</key>
-	<true/>
 </dict>
 </plist>