[Config] Mark two 'RCNConfigSettings' properties atomic

Author: ncooke3

FirebaseRemoteConfig/Sources/Private/RCNConfigSettings.h

@@ -41,9 +41,11 @@
 /// Device data version of checkin information.
 @property(nonatomic, copy) NSString *deviceDataVersion;
 /// InstallationsID.
-@property(nonatomic, copy) NSString *configInstallationsIdentifier;
+/// @note The property is atomic because it is access across multiple threads.
+@property(atomic, copy) NSString *configInstallationsIdentifier;
 /// Installations token.
-@property(nonatomic, copy) NSString *configInstallationsToken;
+/// @note The property is atomic because it is access across multiple threads.
+@property(atomic, copy) NSString *configInstallationsToken;
 
 /// A list of successful fetch timestamps in milliseconds.
 /// TODO Not used anymore. Safe to remove.