[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