Fix mutating array during iteration crash

[danielamitay]

@paulb777 @morganchen12 @karenyz Please take a look! This PR is a follow-up to #11379

The above linked PR fixes iterating over an array that can be mutated simultaneously elsewhere, but it only fixes it for the experiments array. I have been seeing crashes in my app using FirebaseABTesting on 10.12.0, but for the payloads array.

This file has three locations that iterate over payloads, and the two other locations do [payloads copy] to avoid this problem, and this is the last callsite of payloads in this file without a matching copy to avoid this crash.

Fixes:

Text:

Fatal Exception: NSGenericException
0  CoreFoundation                 0x9cb4 __exceptionPreprocess
1  libobjc.A.dylib                0x183d0 objc_exception_throw
2  CoreFoundation                 0x176920 -[__NSSingleObjectEnumerator init]
3  <redacted>                     0x436a0 ABTExperimentsToClearFromPayloads + 95 (FIRExperimentController.m:95)
4  <redacted>                     0x43db8 -[FIRExperimentController updateExperimentConditionalUserPropertiesWithServiceOrigin:events:policy:lastStartTime:payloads:completionHandler:] + 216 (FIRExperimentController.m:216)
5  <redacted>                     0x43c30 __117-[FIRExperimentController updateExperimentsWithServiceOrigin:events:policy:lastStartTime:payloads:completionHandler:]_block_invoke + 180 (FIRExperimentController.m:180)
6  libdispatch.dylib              0x2320 _dispatch_call_block_and_release
7  libdispatch.dylib              0x3eac _dispatch_client_callout
8  libdispatch.dylib              0x15a64 _dispatch_root_queue_drain
9  libdispatch.dylib              0x16158 _dispatch_worker_thread2
10 libsystem_pthread.dylib        0xda0 _pthread_wqthread
11 libsystem_pthread.dylib        0xb7c start_wqthread

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[paulb777]

Thanks @danielamitay 💯