Genera un BillingClient

Crea un singleton BillingClient completo e solido con listener e una policy di nuovi tentativi di connessione.

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