Stories: Fix StringIndexOutOfBoundsException when below another block

[aforcier]

Fixes Automattic/stories-android#615.

It turns out that the StringIndexOutOfBoundsException was happening when the Story block was not the first block in the post. The end of the first block in the post would be matched, causing the substring end index to be earlier than the start index.

Fixed by calculating the end of the Story block relative to its start.

To test:

• Create a new post in Gutenberg
• Add a block, e.g. a paragraph block
• Below it, add a Story block, and add some slides
• Exit the story editor and try to save the post
• Confirm that the post is saved and there is no crash

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

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You can test the changes on this Pull Request by downloading the APK here.

[mzorz]

Good catch @aforcier ! Excellent, verified it works and yep, it makes sense 👍
LGTM