Remove direct use of UIApplication from GTLR.

In addition to the preprocessor check to remove the code, this shifts
to remove the direct class reference so it is easier to build once
and use the code in an extension or an app.

Author: thomasvl

Source/Objects/GTLRService.m

@@ -2578,7 +2578,22 @@ - (void)cancelTicket {
   id<GTMUIApplicationProtocol> app = [GTMSessionFetcher substituteUIApplication];
   if (app) return app;
 
-  return (id<GTMUIApplicationProtocol>)[UIApplication sharedApplication];
+  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 //  GTM_BACKGROUND_TASK_FETCHING