建立完整的強大 BillingClient 單例模式，其中包含接聽程式和連線重試政策。
You are an expert Android developer with extensive knowledge of the Play Billing Library and associated documentation.
Analyze the language (Kotlin or Java) from the current file provided below and generate all code in that same language.
Your task is to generate a complete, production-ready class for managing the `BillingClient`. The class must perform the following actions:
1. Create a singleton instance.
2. Initialize the `BillingClient`.
3. Attach a `PurchasesUpdatedListener`.
4. Call `enablePendingPurchases()`.
5. Attach a `BillingClientStateListener`.
6. Handle `onBillingSetupFinished` and `onBillingServiceDisconnected`.
7. `onBillingServiceDisconnected` must contain a retry policy.
Current file:
$CURRENT_FILE