Remove SystemState from post-install step.

The post-install action only requires to mark the new slot as ready,
for which it was including a reference to the whole SystemState. This
patch removes said dependency replacing it for just the
BootControlInterface.

Bug: 25773375
TEST=FEATURES=test emerge-link update_engine; mma

Change-Id: I814d47c138c7565e9a80f316f25e124adb0d9c4e
diff --git a/update_attempter.cc b/update_attempter.cc
index 4790e84..3cdd613 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -564,7 +564,7 @@
 void UpdateAttempter::BuildPostInstallActions(
     InstallPlanAction* previous_action) {
   shared_ptr<PostinstallRunnerAction> postinstall_runner_action(
-      new PostinstallRunnerAction(system_state_));
+      new PostinstallRunnerAction(system_state_->boot_control()));
   actions_.push_back(shared_ptr<AbstractAction>(postinstall_runner_action));
   BondActions(previous_action,
               postinstall_runner_action.get());