adding an index on DateTakens table

app/src/main/kotlin/com/simplemobiletools/gallery/pro/databases/GalleryDatabase.kt

@@ -65,6 +65,7 @@ abstract class GalleryDatabase : RoomDatabase() {
         private val MIGRATION_6_7 = object : Migration(6, 7) {
             override fun migrate(database: SupportSQLiteDatabase) {
                 database.execSQL("CREATE TABLE IF NOT EXISTS `date_takens` (`id` INTEGER PRIMARY KEY AUTOINCREMENT, `full_path` TEXT NOT NULL, `parent_path` TEXT NOT NULL, `last_fixed` INTEGER NOT NULL)")
+                database.execSQL("CREATE UNIQUE INDEX `index_date_takens_full_path` ON `date_takens` (`full_path`)")
             }
         }
     }