Поиск стикеров Gboard не дает результата

Проблема в другой части кода, потому что то, что вы разместили выше, отлично работает в следующем примере, адаптированном из пример проекта Google для наклеек:

public static void clearStickers(FirebaseAppIndex firebaseAppIndex) {
    Task<Void> task = firebaseAppIndex.removeAll();
    task.addOnSuccessListener(__ -> Log.i(TAG, SUCCESS_IN_CLEARING_STICKERS));
    task.addOnFailureListener(e -> Log.i(TAG, FAILED_TO_CLEAR_STICKERS, e));
}

public static void setStickers(final Context context, FirebaseAppIndex firebaseAppIndex) {
    try {
        List<Indexable> stickers = getIndexableStickers(context);
        Indexable stickerPack = getIndexableStickerPack(context);

        List<Indexable> indexables = new ArrayList<>(stickers);
        indexables.add(stickerPack);

        Task<Void> task = firebaseAppIndex.update(
                indexables.toArray(new Indexable[indexables.size()]));
        task.addOnSuccessListener(__ -> Log.i(TAG, SUCCESS_IN_INSTALLING_STICKERS));
        task.addOnFailureListener(e -> Log.i(TAG, FAILED_TO_INSTALL_STICKERS, e));

    } catch (IOException | FirebaseAppIndexingInvalidArgumentException e) {
        Log.e(TAG, ERROR_IN_SETTING_STICKERS, e);
    }
}

private static Indexable getIndexableStickerPack(final Context context)
        throws IOException, FirebaseAppIndexingInvalidArgumentException {
    List<Indexable> indexables = StreamSupport
            .stream(getStickerBuilders(context))
            .map(IndexableBuilder::build)
            .collect(Collectors.toList());

    StickerPackBuilder stickerPackBuilder = Indexables.stickerPackBuilder()
            .setName(STICKER_PACK_NAME)
            // Use the first sticker as the hero for the pack
            .setUrl(String.format(STICKER_PACK_URL_PATTERN, 0))
            // Defaults to the first sticker in "hasSticker". Used to select between sticker
            // packs so should be representative of the sticker pack.
            .setImage(StickersDataFactory.getAllStickerReference().get(0).getURL())
            .put("hasSticker", indexables.toArray(new Indexable[indexables.size()]))
            .setDescription(STICKER_PACK_NAME);
    return stickerPackBuilder.build();
}

private static List<Indexable> getIndexableStickers(final Context context)
        throws IOException, FirebaseAppIndexingInvalidArgumentException {
    List<Indexable> indexableStickers = new ArrayList<>();
    List<Indexable.Builder> stickerBuilders = getStickerBuilders(context);

    for (Indexable.Builder stickerBuilder : stickerBuilders) {
        stickerBuilder.put("keywords", getKeywordsForSticker(context, null))
                .put("isPartOf", new Indexable.Builder("StickerPack").setName(STICKER_PACK_NAME) .build());

        indexableStickers.add(stickerBuilder.build());
    }

    return indexableStickers;
}

private static List<Indexable.Builder> getStickerBuilders(final Context context)
        throws IOException, FirebaseAppIndexingInvalidArgumentException {
    List<Indexable.Builder> stickerBuilders = new ArrayList<>();

    // Start from 1 because the first sticker will be used as hero for the pack
    for (int i = 1; i < StickersDataFactory.getAllStickerReference().size(); i++) {
            stickerBuilders.add(new Indexable.Builder("Sticker")
                    .setName(StickersDataFactory.getAllStickerReference().get(i).getName())
                    .setUrl(String.format(STICKER_URL_PATTERN, i))
                    .setImage(StickersDataFactory.getAllStickerReference().get(i).getURL())
                    .setDescription("Random description")
                    .put("keywords", getKeywordsForSticker(context, StickersDataFactory.getAllStickerReference().get(i)))
                    .put("isPartOf", new Indexable.Builder("StickerPack").setName(STICKER_PACK_NAME) .build()));
    }

    return stickerBuilders;
}

Вместо new Indexable.Builder("Sticker") должно быть Indexables.stickerBuilder(). Я взял это из этого официального Google пример.