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/mock_dynamic_partition_control.h b/mock_dynamic_partition_control.h
index 1237d76..b3e0c24 100644
--- a/mock_dynamic_partition_control.h
+++ b/mock_dynamic_partition_control.h
@@ -44,6 +44,11 @@
   MOCK_METHOD0(GetVirtualAbFeatureFlag, FeatureFlag());
   MOCK_METHOD0(FinishUpdate, bool());
   MOCK_METHOD0(CleanupSuccessfulUpdate, ErrorCode());
+  MOCK_METHOD3(GetCleanupPreviousUpdateAction,
+               std::unique_ptr<AbstractAction>(
+                   BootControlInterface*,
+                   PrefsInterface*,
+                   CleanupPreviousUpdateActionDelegateInterface*));
 };
 
 class MockDynamicPartitionControlAndroid