Move FinishedSnapshotWrites call back to PostinstallRunnerAction

After FinishedSnapshotWrites is called, if device reboots w/o slot
switch, libsnapshot will discard all update state. This makes slot
switching after reboot difficult. For better UX, move back.

This reverts commit 5b00dc5386e67db5609e79f9e5c603d4ef1098a9.

Reason for revert: b/318986391

Change-Id: If33d7661a907d779f1a860b439a707885a8882bf
diff --git a/payload_consumer/postinstall_runner_action.h b/payload_consumer/postinstall_runner_action.h
index 66721af..6017069 100644
--- a/payload_consumer/postinstall_runner_action.h
+++ b/payload_consumer/postinstall_runner_action.h
@@ -42,6 +42,7 @@
  public:
   PostinstallRunnerAction(BootControlInterface* boot_control,
                           HardwareInterface* hardware);
+  ~PostinstallRunnerAction();
 
   // InstallPlanAction overrides.
   void PerformAction() override;