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/update_attempter_android.h b/update_attempter_android.h
index 44b66d3..309adff 100644
--- a/update_attempter_android.h
+++ b/update_attempter_android.h
@@ -77,6 +77,10 @@
   bool ResetStatus(brillo::ErrorPtr* error) override;
   bool VerifyPayloadApplicable(const std::string& metadata_filename,
                                brillo::ErrorPtr* error) override;
+  uint64_t AllocateSpaceForPayload(
+      const std::string& metadata_filename,
+      const std::vector<std::string>& key_value_pair_headers,
+      brillo::ErrorPtr* error) override;
 
   // ActionProcessorDelegate methods:
   void ProcessingDone(const ActionProcessor* processor,