Fix build for app extensions

[vegaro]

Customer Center broke the build for app extensions, this PR brings an approach similar to

purchases-ios/Sources/Support/ManageSubscriptionsHelper.swift

Lines 107 to 109 in 98e4a33

	// we can't directly reference UIApplication.shared in case this SDK is embedded into an app extension.
	// so we ensure that it's not running in an app extension and use selectors to call UIApplication methods.
	func openURLIfNotAppExtension(url: URL) {

to the Customer Center

[tonidero]

LGTM!