[Config] Mark two RCNConfigSettings properties atomic (#13925)

Author: ncooke3

FirebaseRemoteConfig/CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+- [fixed] Mark two internal properties as `atomic` to prevent concurrency
+  related crash. (#13898)
+
 # 11.0.0
 - [fixed] RemoteConfigValue stringValue is now `nonnull`. This may break some builds. (#10870)
 - [removed] **Breaking change**: The deprecated `FirebaseRemoteConfigSwift`

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 accessed across multiple threads.
+@property(atomic, copy) NSString *configInstallationsIdentifier;
 /// Installations token.
-@property(nonatomic, copy) NSString *configInstallationsToken;
+/// @note The property is atomic because it is accessed across multiple threads.
+@property(atomic, copy) NSString *configInstallationsToken;
 
 /// A list of successful fetch timestamps in milliseconds.
 /// TODO Not used anymore. Safe to remove.