doc(android): refactor build-extras.gradle

[erisu]

Motivation and Context

Cleanup around Android's build-extras.gradle.

Description

• Refactored wording
• Replaced hook script step with <resource-file>
• Moved the plugin developer note higher.

Testing

n/a

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary