Migrate Provide custom back navigation snippets

[hamen]

Code snippets are for:

• https://developer.android.com/guide/navigation/custom-back

List of modifications:

• Custom-back page: Migrated Compose BackHandler basic usage snippet to compose/snippets/src/main/java/com/example/compose/snippets/navigation/CustomBack.kt (region tag android_compose_navigation_custom_back_basic). Spotless line delta: 0 lines modified.

Snippets not migrated:

• MyFragment OnBackPressedCallback snippet because it is View-based (Fragment/OnBackPressedDispatcher) and excluded per DAC snippet eligibility rules; the obsolete Java tab was removed per D15.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• compose/snippets/src/main/java/com/example/compose/snippets/navigation/CustomBack.kt:26, tag android_compose_navigation_custom_back_basic

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment