Support for Lesson Progress Study Guides and Worked Examples

The Oppia Android app is missing three features that the web platform already has. Learners have no idea how far they are in a lesson, revision cards are a flat wall of text with no structure, and worked examples in concept cards are completely invisible. This project adds all three. First, a checkpoint based lesson progress indicator that uses the is_checkpoint state property to show learners their progress as X of N through an exploration, with correct behavior during backward navigation, flashbacks, and checkpoint restoring. Second, structured study guides that replace the flat revision card with a sectioned RecyclerView showing headings and content blocks. Third, a worked example tag handler that parses nested HTML inside oppia-noninteractive-workedexample tags and renders inline Question and Answer blocks using styled spannables. All three features are independently gated behind feature flags and built on top of existing infrastructure.