Add billing module and include in play implementation.

app/src/main/java/org/thoughtcrime/securesms/billing/GooglePlayBillingApi.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.billing
+
+/**
+ * Variant interface for the BillingApi.
+ */
+interface GooglePlayBillingApi {
+  fun isApiAvailable(): Boolean = false
+  suspend fun queryProducts() {}
+
+  /**
+   * Empty implementation, to be used when play services are available but
+   * GooglePlayBillingApi is not available.
+   */
+  object Empty : GooglePlayBillingApi
+}