Add droidstubs support for --validate-nullability-from-list.

This names an input file for the droidstubs rule, so needs to be
handled in soong.

Test: core-current-stubs-nullability-validation-check-nullability-warnings
Bug: 73448108
Change-Id: I092c89f01c8a448ca486e4b57d43a1c3af133114