update_engine: Block force update + install when not idle

Currently |CheckForUpdate()| and |CheckForInstall()| sets the values of
|is_install_| to false and true respectively. The change of the value
|is_install_| at the initial entry to these functions will cause the
checks and actions taken within |ProcessingDone()| to behave as it's not
intended to. This unwanted behavior occurs because |ProcessingDone()|
depends on |UpdateAttempter|'s member variable |is_install_|. Until
|ProcessingDone()| is called, the |is_install_| value needs to remain
the same.

BUG=chromium:982929
TEST=FEATURES="test" emerge-$BOARD update_engine

Change-Id: Iddb55f1b46e66b9bc94b63e10e9f393f3cb89e1c
diff --git a/update_attempter.h b/update_attempter.h
index b0654d8..3b580ad 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -250,6 +250,7 @@
   FRIEND_TEST(UpdateAttempterTest, BootTimeInUpdateMarkerFile);
   FRIEND_TEST(UpdateAttempterTest, BroadcastCompleteDownloadTest);
   FRIEND_TEST(UpdateAttempterTest, ChangeToDownloadingOnReceivedBytesTest);
+  FRIEND_TEST(UpdateAttempterTest, CheckForInstallNotIdleFails);
   FRIEND_TEST(UpdateAttempterTest, CheckForUpdateAUDlcTest);
   FRIEND_TEST(UpdateAttempterTest, CreatePendingErrorEventTest);
   FRIEND_TEST(UpdateAttempterTest, CreatePendingErrorEventResumedTest);