UpdateAttempterAndroid::Init initiates merge

On update_engine starts, schedule CleanupPreviousUpdateAction that calls
CleanupSuccessfulUpdate to do necessary cleanup as soon as possible.

In the good case, update_engine initiates merge when
sys.boot_completed, and clean up snapshots.

If the update is
rolled back or partitions are flashed, the following happens (on
a Virtual A/B device):

- UpdateAttempterAndroid::CleanupSuccessfulUpdate is called
  - DynamicPartitionControlAndroid::CleanupSuccessfulUpdate is called
  - SnapshotManager::InitiateMergeAndWait is called
  - SnapshotManager::RemoveAllUpdateState(before_cancel) is called
  - before_cancel is called,
    DeltaPerformer::ResetUpdateProgress is called
    - All update states in update_engine is reset.
  - SnapshotManager proceeds to delete snapshots
    - All update states in SnapshotManager is reset.

Hence, on an VAB device, when an update is rolled back or partitions
are flashed, the whole update needs to be re-applied
(while in A/B, it skips writing and directly start verifying hashes of
the target partitions because the update markers are still there).

Bug: 147696014
Test: apply OTA then reboot, inspect logs and do `snapshotctl dump`

Change-Id: I0fc5e7768dfb53e4fd474f2d8d85d2a1b615a88b
diff --git a/common/action.h b/common/action.h
index 9e2f5ff..c93e73c 100644
--- a/common/action.h
+++ b/common/action.h
@@ -222,6 +222,16 @@
       out_pipe_;
 };
 
+// An action that does nothing and completes with kSuccess immediately.
+class NoOpAction : public AbstractAction {
+ public:
+  ~NoOpAction() override {}
+  void PerformAction() override {
+    processor_->ActionComplete(this, ErrorCode::kSuccess);
+  }
+  std::string Type() const override { return "NoOpAction"; }
+};
+
 };  // namespace chromeos_update_engine
 
 #endif  // UPDATE_ENGINE_COMMON_ACTION_H_