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>