Fix `AlertState.init` availability #456

Job	Run time
Windows (windows-latest, debug)	5m 16s
library (15.4, ios)	6m 24s
Windows (windows-latest, release)	13m 57s
library (15.4, macos)	5m 10s
library (15.4, tvos)	8m 57s
library (15.4, watchos)	6m 36s
library (15.4, examples)	8m 13s
	54m 33s

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