Turn on nullability annotation for the non-ojluni classes.

Previously, libcore does nullability validation of ojluni classes,
using the existence of nullability stub files to indicate which
classes to validate. This change turns on validation for classes which
are annotated directly in the source rather than using stubs, by
listing them explicitly.

This change also adds one annotation previously missing from
BigInteger (which was found by this validation).

Test: core-current-stubs-nullability-validation-check-nullability-warnings
Bug: 73448108
Bug: 64930165
Change-Id: I21fcf45c4c58ec9b93333efbdd29de3eca2f3e81
3 files changed