update_engine: UM: UpdateCheckAllowed returns target version prefix.

This piece of information is also provided by the device policy and
needed when performing the update check. Borrows logic currently in
UpdateAttempter::CalculateUpdateParams().

BUG=chromium:358269
TEST=Unit tests.

Change-Id: I5259404df1763be8944b445d51e1fafbca946306
Reviewed-on: https://chromium-review.googlesource.com/209228
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/update_manager/default_policy.cc b/update_manager/default_policy.cc
index 91b1bba..98c2786 100644
--- a/update_manager/default_policy.cc
+++ b/update_manager/default_policy.cc
@@ -24,6 +24,7 @@
     UpdateCheckParams* result) const {
   result->updates_enabled = true;
   result->target_channel.clear();
+  result->target_version_prefix.clear();
   result->is_interactive = false;
 
   // Ensure that the minimum interval is set. If there's no clock, this defaults