Remove direct use of UIApplication from GTMSessionFetcher.

iOS application extensions don't play well with +[UIApplication sharedApplication],
and attempting to compile the symbol gets a compile error. We previously used
GTM_BACKGROUND_UIAPPLICATION to gate access, but that doesn't work when building
shared frameworks to use with both the app and an extension.

This removes the GTM_BACKGROUND_UIAPPLICATION define altogether. It does leave
GTM_BACKGROUND_TASK_FETCHING, in case clients want to compile out the background
task behavior altogether.

Source/GTMSessionFetcher.h

@@ -382,13 +382,6 @@
   #define GTM_BACKGROUND_TASK_FETCHING 1
 #endif
 
-// If GTM_BACKGROUND_TASK_FETCHING is enabled and GTMUIApplicationProtocol is not used,
-// GTM_BACKGROUND_UIAPPLICATION will allow defaulting to UIApplication. To avoid references to
-// UIApplication (e.g. for extensions), set GTM_BACKGROUND_UIAPPLICATION to 0.
-#if TARGET_OS_IPHONE && !TARGET_OS_WATCH && !defined(GTM_BACKGROUND_UIAPPLICATION)
-  #define GTM_BACKGROUND_UIAPPLICATION 1
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif

Source/GTMSessionFetcher.m

@@ -1977,13 +1977,25 @@ + (void)setSubstituteUIApplication:(nullable id<GTMUIApplicationProtocol>)app {
   id<GTMUIApplicationProtocol> app = gSubstituteUIApp;
   if (app) return app;
 
-  // Some projects use GTM_BACKGROUND_UIAPPLICATION to avoid compile-time references
-  // to UIApplication.
-#if GTM_BACKGROUND_UIAPPLICATION
-  return (id<GTMUIApplicationProtocol>) [UIApplication sharedApplication];
-#else
-  return nil;
-#endif
+  // iOS App extensions should not call [UIApplication sharedApplication], even
+  // if UIApplication responds to it.
+
+  static Class applicationClass = nil;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    BOOL isAppExtension = [[[NSBundle mainBundle] bundlePath] hasSuffix:@".appex"];
+    if (!isAppExtension) {
+      Class cls = NSClassFromString(@"UIApplication");
+      if (cls && [cls respondsToSelector:NSSelectorFromString(@"sharedApplication")]) {
+        applicationClass = cls;
+      }
+    }
+  });
+
+  if (applicationClass) {
+    app = (id<GTMUIApplicationProtocol>)[applicationClass sharedApplication];
+  }
+  return app;
 }
 #endif //  TARGET_OS_IPHONE