Internal properties of state object should be private.

[DubeySandeep]

All over the test files the state objects are updated directly accessing the object's internal properties but from the top level, we use update_interaction_hints, update_interaction_answer_groups etc. to update the internal properties of the state, so making those internal properties private and updating the test file to follow a similar pattern would make a consitent and clean tests.

Note: Linking content_id to assets is managed internally and also from this perspective we expect all test to use a top-level method to update the internal properties such that they will go through updating assets with the content ids.

see this comment for reference.

[oppiabot]

Hi @oppia/core-maintainers, this issue is not assigned to any project. Can you please update the same? Thanks!