[Vertex AI] Refactor `ImagenSafetySettings` #839

Job	Run time
pod-lib-lint (ios, macos-14)	8m 21s
spm-package-resolved	1m 16s
sample (macos-13, Xcode_15.2)	3m 7s
spm-unit (macos-13, Xcode_15.2, iOS)	6m 14s
sample (macos-14, Xcode_15.4)	2m 43s
testapp-integration (iOS, macos-15)	2m 2s
spm-unit (macos-14, Xcode_15.4, iOS)	2m 7s
sample (macos-15, Xcode_16)	2m 8s
spm-unit (macos-15, Xcode_16, iOS)	2m 18s
spm-unit (macos-15, Xcode_16, tvOS)	1m 54s
spm-unit (macos-15, Xcode_16, macOS)	1m 56s
spm-unit (macos-15, Xcode_16, watchOS)	2m 33s
spm-unit (macos-15, Xcode_16, catalyst)	1m 48s
spm-unit (macos-15, Xcode_16, visionOS)	3m 56s
	42m 23s

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