要提供超过90天的免费年度订阅促销/优惠，您可以使用Google Play Billing Library来实现。您需要在应用中集成此库，并按照以下步骤执行：

1. 在您的应用级build.gradle文件中添加以下依赖项：

dependencies {
    implementation 'com.android.billingclient:billing:4.0.0'
}

2. 在您的代码中，创建一个BillingClient实例：

private BillingClient billingClient;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    billingClient = BillingClient.newBuilder(this)
            .setListener(purchaseUpdateListener)
            .enablePendingPurchases()
            .build();
    
    billingClient.startConnection(new BillingClientStateListener() {
        @Override
        public void onBillingSetupFinished(BillingResult billingResult) {
            if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                // 连接成功，可以进行购买操作
            }
        }

        @Override
        public void onBillingServiceDisconnected() {
            // 连接中断，可以在此处处理重连逻辑
        }
    });
}

3. 实现PurchaseUpdateListener接口，用于处理购买结果和订阅状态更新：

private PurchaseUpdateListener purchaseUpdateListener = new PurchaseUpdateListener() {
    @Override
    public void onPurchasesUpdated(BillingResult billingResult, List purchases) {
        if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK && purchases != null) {
            for (Purchase purchase : purchases) {
                // 处理购买成功的逻辑
                if (purchase.getPurchaseState() == Purchase.PurchaseState.PURCHASED) {
                    // 购买成功
                }
                // 处理订阅状态更新的逻辑
                else if (purchase.getPurchaseState() == Purchase.PurchaseState.PENDING) {
                    // 订阅状态未确定
                }
                else if (purchase.getPurchaseState() == Purchase.PurchaseState.UNSPECIFIED_STATE) {
                    // 订阅状态未知
                }
            }
        }
        else if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.USER_CANCELED) {
            // 用户取消购买
        }
        else {
            // 购买失败
        }
    }
};

4. 发起购买请求：

String skuId = "your_sku_id"; // 替换为您的产品SKU ID

String[] skuList = new String[]{skuId};
List skuList = Arrays.asList(skuIds);

SkuDetailsParams params = SkuDetailsParams.newBuilder()
        .setSkusList(skuList)
        .setType(BillingClient.SkuType.SUBS)
        .build();

billingClient.querySkuDetailsAsync(params, new SkuDetailsResponseListener() {
    @Override
    public void onSkuDetailsResponse(BillingResult billingResult, List skuDetailsList) {
        if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK && skuDetailsList != null) {
            // 获取到产品详情
            for (SkuDetails skuDetails : skuDetailsList) {
                String sku = skuDetails.getSku();
                String price = skuDetails.getPrice();
                // 显示产品详情，并让用户确认购买
            }
        }
    }
});

billingClient.launchBillingFlow(MainActivity.this, billingFlowParams);

这些代码示例演示了如何使用Google Play Billing Library实现订阅促销/优惠。您可以根据您的具体需求进行修改和扩展。请确保您已在Google Play开发者控制台中正确设置了相关的订阅产品和促销设置。