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

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>

(cherry picked from commit 4d34c18e5ce29d138c7946cc817dee8b3fce7cf0)

Change-Id: I712b169505f4566aac3a0b2c9746dfcb9466ef8d
diff --git a/update_manager/chromeos_policy.cc b/update_manager/chromeos_policy.cc
index ec2b9f0..ffd378c 100644
--- a/update_manager/chromeos_policy.cc
+++ b/update_manager/chromeos_policy.cc
@@ -245,9 +245,9 @@
       }
 
       result->target_version_prefix = *kiosk_required_platform_version_p;
-      LOG(INFO) << "Allow kiosk app to control Chrome version policy is set,"
-                << ", target version is "
-                << (kiosk_required_platform_version_p
+      LOG(INFO) << "Allow kiosk app to control Chrome version policy is set, "
+                << "target version is "
+                << (!kiosk_required_platform_version_p->empty()
                         ? *kiosk_required_platform_version_p
                         : std::string("latest"));
     } else {