Camera: Add framework-only enum value to test pattern mode

Adding TEST_PATTERN_MODE_BLACK to support the new sensor privacy
feature on more limited-capability camera implementations.

Also updating older docs due to an API being moved to public.

Test: Camera CTS continues to pass
Bug: 189507251
Change-Id: I043d011ad43070d6c402cdffbff3d2e9b70ffd7b