Explanation

Fixes #2476
Fixes #3312
Fixes #3703

Note that this PR was made in collaboration between @MaskedCarrot, @yashraj-01, and @BenHenning (see commits for specifics). Further, much of this was built as part of #3696 as a separate rewrite to address specific technical needs for both downstream dependent projects (lightweight checkpointing & developer options menu).

At a high-level, this PR moves & rebuilds HintHandler from the UI layer to being a separate, independent logic unit within the domain layer. Key changes that are meant to simplify hint handling:

1. HintHandler was moved to be an interface so that multiple implementations can be provided
2. HelpIndex is now the singular source of truth for all hint/solution state everywhere (domain, checkpoint, and UI); the previous revealed states in Hint/Solution have been removed (which now means those structures are properly immutable for the lifetime of a play session)
3. HintHandler is responsible for everything hints & solution (when there are new ones available, and managing when they've been shown) by being the sole provider of HelpIndex. This includes both computing the available hints/solution as well as managing when hints & the solution have been revealed. This has led to lots of simplifications in the UI.
4. HintHandler's API has been reinvented to be a bit more operation-specific and less generic
5. A monitor was introduced for HintHandler so that the calling controllers know when to re-query for the latest state. There's definitely a data race here, but the data model is eventually consistent so it should be fine.
6. HintHandler's tests make use of lots of new highly specific explorations. These explorations are not accessible from the topic list; they are only included for testing purposes (and are meant to exemplars for future tests).

This PR introduces a few other examples for future changes:

• Module-specific test to verify bindings
• Extensions class tests

While neither of those are unique, the relevant part is that these were the default approaches taken to address the test file presence requirement.

Other details:

• This also fixes Non-modularized testing module tests not being built in Bazel today #3703 since it was found in passing that //testing module tests were not being built or run in Bazel (except those that are already modularized), leading to some build errors that weren't caught by the CI environment as they should have been.
• QuestionAssessmentProgressControllerTest has been updated to include new scores since it was previously testing impossible scenarios (i.e. assuming hints could be shown/revealed when they wouldn't actually show up)

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The PR explanation includes the words "Fixes #bugnum: ..." (or "Fixes part of #bugnum" if the PR only partially fixes an issue).
• The PR follows the style guide.
• The PR does not contain any unnecessary auto-generated code from Android Studio.
• The PR is made from a branch that's not called "develop".
• The PR is made from a branch that is up-to-date with "develop".
• The PR's branch is based on "develop" and not on any other branch.
• The PR is assigned to an appropriate reviewer in both the Assignees and the Reviewers sections.

Without this fix, tests (added in this PR because they were not present) fail on the latest develop.