[Config] Mark two RCNConfigSettings properties atomic

[ncooke3]

The crashing frame is on objc_release_x0 so I'm thinking that that the instance is being over-released.

Thread 24 Queue 295: com.google.GoogleConfigService.FIRRemoteConfig (serial) [Crashed]: 0 libobjc.A.dylib 0x1897da370 objc_release_x0 + 16 1 CoreFoundation 0x18c495394 __CFBasicHashReplaceValue + 415 2 CoreFoundation 0x18c493d04 CFDictionarySetValue + 207 3 CFNetwork 0x18d9409bc HTTPHeaderDict::setValue(HTTPHeaderKeyMixedValue const&, HTTPHeaderValueMixedValue const&) + 143 4 CFNetwork 0x18d94082c HTTPMessage::setHeaderFieldStringValue(__CFString const*, __CFString const*) + 99 5 CFNetwork 0x18d93fbcc CFURLRequestSetHTTPHeaderFieldValue + 127 6 FlexNetworkingProduction 0x101729cb4 __53-[RCNConfigRealtime createRequestBodyWithCompletion:]_block_invoke + 183 7 libdispatch.dylib 0x194164370 _dispatch_call_block_and_release + 31 8 libdispatch.dylib 0x1941660d0 _dispatch_client_callout + 19 9 libdispatch.dylib 0x19416d6d8 _dispatch_lane_serial_drain + 743 10 libdispatch.dylib 0x19416e1e0 _dispatch_lane_invoke + 379 11 libdispatch.dylib 0x194179258 _dispatch_root_queue_drain_deferred_wlh + 287 12 libdispatch.dylib 0x194178aa4 _dispatch_workloop_worker_thread + 539 13 libsystem_pthread.dylib 0x213857c7c _pthread_wqthread + 287 14 libsystem_pthread.dylib 0x213854488 start_wqthread + 7 

These properties are never sent in RCNConfigSettings's initialization but rather on one of two different queues:

firebase-ios-sdk/FirebaseRemoteConfig/Sources/RCNConfigFetch.m

Lines 302 to 311 in 8328630

	// Dispatch to the RC serial queue to update settings on the queue.
	dispatch_async(strongSelf->_lockQueue, ^{
	RCNConfigFetch *strongSelfQueue = weakSelf;
	if (strongSelfQueue == nil) {
	return;
	}
	// Update config settings with the IID and token.
	strongSelfQueue->_settings.configInstallationsToken = tokenResult.authToken;
	strongSelfQueue->_settings.configInstallationsIdentifier = identifier;

firebase-ios-sdk/FirebaseRemoteConfig/Sources/RCNConfigRealtime.m

Lines 263 to 272 in 8328630

	// Dispatch to the RC serial queue to update settings on the queue.
	dispatch_async(strongSelf->_realtimeLockQueue, ^{
	RCNConfigRealtime *strongSelfQueue = weakSelf;
	if (strongSelfQueue == nil) {
	return;
	}
	/// Update config settings with the IID and token.
	strongSelfQueue->_settings.configInstallationsToken = tokenResult.authToken;
	strongSelfQueue->_settings.configInstallationsIdentifier = identifier;

They are currently marked nonatomic, so my hypothesis is that concurrent sets lead to an object with an invalid retain count, and when the property is later accessed here. It causes a crash when trying to over-release the invalid instance.

I did some searching and atomic seemed to be an acceptable solution. Others were to add a queue inside RCNConfigSettings which seemed too heavyweight, as well as overwriting the getter and setter to use @synchronized which seemed reasonable but adding atomic was less LOC.

Fix #13898