Follow up on AllowKioskAppControlChromeVersion policy CL

- Update header includes, adding missed and removing unused.
- UpdateCheckAllowedKioskPinWithNoRequiredVersion verifies target
  version prefix is empty.
- Change RealSystemProvider's required platform version variable to
  be a RetryPollVariable to handle D-Bus call failure.
- Update ChromeOSPolicy to defer update check when kiosk pin policy
  is effective but system provider does not set the required platform
  version.

BUG=chromium:577783

Change-Id: Id5400d09fff69001a37e367fff9ede1664c2e060
diff --git a/update_manager/real_system_provider.h b/update_manager/real_system_provider.h
index d6c2e24..083943b 100644
--- a/update_manager/real_system_provider.h
+++ b/update_manager/real_system_provider.h
@@ -64,7 +64,8 @@
   }
 
  private:
-  std::string GetKioskAppRequiredPlatformVersion();
+  bool GetKioskAppRequiredPlatformVersion(
+      std::string* required_platform_version);
 
   std::unique_ptr<Variable<bool>> var_is_normal_boot_mode_;
   std::unique_ptr<Variable<bool>> var_is_official_build_;