feat: adds a SharedPreferencesMigration example (preferences)

[telpirion]

This PR adds the android_datastore_preferences_migrate region tag. Internal issue 446749253 tracks.

Note: I will add a corresponding snippet for proto & JSON preferences as I have time. Thanks!

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• datastore/src/main/java/com/example/datastore/snippets/sharedprefsmigration/PreferencesDataStore.kt:23, tag android_datastore_preferences_migrate

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[gemini-code-assist]

Summary of Changes

Hello @telpirion, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request provides a clear example of how to implement SharedPreferencesMigration when transitioning from the legacy SharedPreferences API to the modern Preferences DataStore in Android applications. The added snippet illustrates the necessary configuration to produce migrations, ensuring a smooth data transfer for users.

Highlights

• SharedPreferences Migration Example: Introduces a new commented-out code example demonstrating how to use SharedPreferencesMigration to migrate existing SharedPreferences data to Preferences DataStore.
• Region Tag: Adds the android_datastore_preferences_migrate region tag to the new migration example code.

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[gemini-code-assist]

Code Review

This PR adds an example for SharedPreferences migration. While the intent is good, the implementation has a few issues. The most critical is a missing import that makes the code snippet incomplete. Additionally, the way the snippet is commented out will likely prevent documentation tools from extracting it correctly. I've also included suggestions for a typo and to follow Kotlin's best practices for constants. Addressing these points will ensure the example is correct, complete, and functional.

[kkuan2011]

LGTM from snippets repo perspective, may want to get Elif to take a look from DataStore perspective