Fix OIDExternalUserAgentIOSCustomBrowser on iOS 10+ #217

Job	Run time
spm-build-test	53s
pod-lib-lint	2m 43s
xcode-project-test (-scheme AppAuth-iOS -destination 'platform=iOS Simulator,name=iPhone 11,OS=16...	2m 44s
pod-lib-lint (--use-libraries)	4m 28s
xcode-project-test (-scheme AppAuth-macOS -destination 'platform=macOS,arch=x86_64' -sdk 'macosx1...	34s
pod-lib-lint (--use-static-frameworks)	2m 49s
xcode-project-test (-scheme AppAuth_macOS -destination 'platform=macOS,arch=x86_64' -sdk 'macosx1...	42s
xcode-project-test (-scheme AppAuth-tvOS -destination 'platform=tvOS Simulator,name=Apple TV,OS=1...	1m 6s
xcode-project-test (-scheme AppAuth_tvOS -destination 'platform=tvOS Simulator,name=Apple TV,OS=1...	58s
xcode-project-test (-scheme AppAuthTV -destination 'platform=tvOS Simulator,name=Apple TV,OS=16.1...	1m 8s
	18m 5s

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