Add stubs for UpdateEngine.AllocateSpaceForPayload

This API preallocates space for a Virtual A/B update.
Right now, it also returns an error (which becomes a
ServiceSpecificException in Java) when space is insufficient. This will
be fixed in a follow up CL.

Test: pass
Bug: 138808058

Change-Id: I587615ba765acb5a52c1918d6a4acc57a95d75f7
diff --git a/binder_service_android.h b/binder_service_android.h
index 0dda93b..5207075 100644
--- a/binder_service_android.h
+++ b/binder_service_android.h
@@ -70,6 +70,10 @@
   android::binder::Status resetStatus() override;
   android::binder::Status verifyPayloadApplicable(
       const android::String16& metadata_filename, bool* return_value) override;
+  android::binder::Status allocateSpaceForPayload(
+      const android::String16& metadata_filename,
+      const std::vector<android::String16>& header_kv_pairs,
+      int64_t* return_value) override;
 
  private:
   // Remove the passed |callback| from the list of registered callbacks. Called