Started querying the app state for the gpu disabled sync switch (#171785)

Started querying the app state for the gpu disabled sync switch when
starting an engine without a view controller

fixes https://github.com/flutter/flutter/issues/166668

Testing: Integration test is not possible since it would require
manipulation of the operating system and killing / restoring the app.
Unit tests would require executing in the background or mocking out
background detection which is pretty contrived and will complicate
engine initialization. The simple cases will be covered with integration
tests like the wide gamut devicelab tests which loads an image at
startup and grabs a screenshot of it.

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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diff --git a/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm b/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm
index 0390306..ad48363 100644
--- a/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm
+++ b/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm
@@ -848,7 +848,11 @@
   );
 
   // Disable GPU if the app or scene is running in the background.
-  self.isGpuDisabled = self.viewController.stateIsBackground;
+  self.isGpuDisabled = self.viewController
+                           ? self.viewController.stateIsBackground
+                           : FlutterSharedApplication.application &&
+                                 FlutterSharedApplication.application.applicationState ==
+                                     UIApplicationStateBackground;
 
   // Create the shell. This is a blocking operation.
   std::unique_ptr<flutter::Shell> shell = flutter::Shell::Create(