Use libsnapshot stub classes

... to avoid crashes in update_engine. The stub classes
does nothing but return a failure for all function
calls, and act as if Virtual A/B is not enabled.

Fixes: 148956645
Test: update_engine_unittests
Change-Id: I48206958aba8d9a71cc7cf49f6b9d5d4f1c8735d
diff --git a/cleanup_previous_update_action.h b/cleanup_previous_update_action.h
index 91e08b0..6f6ce07 100644
--- a/cleanup_previous_update_action.h
+++ b/cleanup_previous_update_action.h
@@ -49,7 +49,7 @@
   CleanupPreviousUpdateAction(
       PrefsInterface* prefs,
       BootControlInterface* boot_control,
-      android::snapshot::SnapshotManager* snapshot,
+      android::snapshot::ISnapshotManager* snapshot,
       CleanupPreviousUpdateActionDelegateInterface* delegate);
 
   void PerformAction() override;
@@ -67,13 +67,13 @@
  private:
   PrefsInterface* prefs_;
   BootControlInterface* boot_control_;
-  android::snapshot::SnapshotManager* snapshot_;
+  android::snapshot::ISnapshotManager* snapshot_;
   CleanupPreviousUpdateActionDelegateInterface* delegate_;
   std::unique_ptr<android::snapshot::AutoDevice> metadata_device_;
   bool running_{false};
   bool cancel_failed_{false};
   unsigned int last_percentage_{0};
-  android::snapshot::SnapshotMergeStats* merge_stats_;
+  android::snapshot::ISnapshotMergeStats* merge_stats_;
 
   void StartActionInternal();
   void ScheduleWaitBootCompleted();