feat: adds a SharedPreferencesMigration example (preferences) (#761)

* feat: adds android_datastore_preferences_migrate snippet

* addresses reviewer comments

* undo changes to PreferencesDataStore.kt

* Apply Spotless

---------

Co-authored-by: telpirion <7399197+telpirion@users.noreply.github.com>

Author: telpirion

datastore/src/main/java/com/example/datastore/snippets/sharedprefsmigration/PreferencesDataStore.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2026 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.datastore.snippets.sharedprefsmigration
+
+import android.content.Context
+import androidx.datastore.preferences.SharedPreferencesMigration
+import androidx.datastore.preferences.preferencesDataStore
+
+// [START android_datastore_preferences_migrate]
+// Be sure to pass the original SharedPreferences name when creating the migration.
+private const val SETTINGS_NAME = "settings"
+
+private val Context.dataStore by preferencesDataStore(
+    name = SETTINGS_NAME,
+    produceMigrations = { context ->
+        listOf(SharedPreferencesMigration(context, SETTINGS_NAME))
+    }
+)
+// [END android_datastore_preferences_migrate]