Add more api calls for billing integration.

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

@@ -5,12 +5,22 @@
 
 package org.thoughtcrime.securesms.billing
 
+import android.app.Activity
+
 /**
  * Variant interface for the BillingApi.
  */
 interface GooglePlayBillingApi {
   fun isApiAvailable(): Boolean = false
-  suspend fun queryProducts() {}
+  suspend fun queryProducts() = Unit
+
+  /**
+   * Queries the user's current purchases. This enqueues a check and will
+   * propagate it to the normal callbacks in the api.
+   */
+  suspend fun queryPurchases() = Unit
+
+  suspend fun launchBillingFlow(activity: Activity) = Unit
 
   /**
    * Empty implementation, to be used when play services are available but