Always update target version prefix

When UpdateAttempter::Update is called, the cached target version
prefix in its |omaha_request_params_| should always be updated with
the one passed in.

Also fix the target version LOG lines in kiosk app pin policy code.

BUG=chromium:695133
TEST=UpdateAttempterTest.TargetVersionPrefixSetAndReset

Change-Id: I873a9ad9151f47414aac9273bb3b85d4f9f73f32
Reviewed-on: https://chromium-review.googlesource.com/446657
Commit-Ready: Xiyuan Xia <xiyuan@chromium.org>
Tested-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/update_attempter_unittest.cc b/update_attempter_unittest.cc
index 99cd2c8..0eaedf5 100644
--- a/update_attempter_unittest.cc
+++ b/update_attempter_unittest.cc
@@ -997,4 +997,14 @@
   EXPECT_EQ(constants::kOmahaDefaultAUTestURL, attempter_.forced_omaha_url());
 }
 
+TEST_F(UpdateAttempterTest, TargetVersionPrefixSetAndReset) {
+  attempter_.CalculateUpdateParams("", "", "", "1234", false, false);
+  EXPECT_EQ("1234",
+            fake_system_state_.request_params()->target_version_prefix());
+
+  attempter_.CalculateUpdateParams("", "", "", "", false, false);
+  EXPECT_TRUE(
+      fake_system_state_.request_params()->target_version_prefix().empty());
+}
+
 }  // namespace chromeos_update_engine