Camera: Add black test pattern mode for privacy

Add an enum value for test pattern BLACK, to be used with
camera muting on devices that can't implement the full
SOLID_COLOR test pattern API.

Also include it in test API to ensure correct behavior can
be tested.

Test: Camera CTS continues to pass
Bug: 189507251
Change-Id: Ie9449f722fdc356c690f1513d4cb25a43a14e4c8