[AppCheck] Fix unused isTokenAutoRefreshEnabled ivar warning (#10818)

Stopped synthesizing a backing instance variable for the `isTokenAutoRefreshEnabled` property in `FIRAppCheckSettings` because it is unused, resulting in a warning when "-Wunused-property-ivar" is enabled.

This resolves the error "FIRAppCheckSettings.m:92:1: error: ivar '_isTokenAutoRefreshEnabled' which backs the property is not referenced in this property's accessor [-Werror,-Wunused-property-ivar]" when building in google3 (where this warning is treated as an error).

Author: andrewheard

FirebaseAppCheck/Sources/Core/FIRAppCheckSettings.m

@@ -31,13 +31,13 @@ @interface FIRAppCheckSettings ()
 @property(nonatomic, readonly) NSUserDefaults *userDefaults;
 @property(nonatomic, readonly) NSBundle *mainBundle;
 @property(nonatomic, readonly) NSString *userDefaultKey;
-
 @property(nonatomic, nullable) NSNumber *isTokenAutoRefreshEnabledNumber;
+
 @end
 
 @implementation FIRAppCheckSettings
 
-@synthesize isTokenAutoRefreshEnabled = _isTokenAutoRefreshEnabled;
+@dynamic isTokenAutoRefreshEnabled;
 
 - (instancetype)initWithApp:(FIRApp *)firebaseApp
                 userDefault:(NSUserDefaults *)userDefaults