update_engine: Multi-level |PrefsInterface::CreateSubKey()|

Currently, |PrefsInterface::CreateSubKey()| is limited to always provide
a namespace and subpref, but this can be generalized to a multi-level
namespace alongside a supplied key.

BUG=chromium:928805
TEST=FEATURES=test emerge-$B update_engine

Change-Id: Ib81e93e8319714caa85cd2fe6495d3cb9b0e82ed
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2195623
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Andrew Lassalle <andrewlassalle@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/omaha_request_action_unittest.cc b/omaha_request_action_unittest.cc
index e1f5ef9..765af4f 100644
--- a/omaha_request_action_unittest.cc
+++ b/omaha_request_action_unittest.cc
@@ -453,11 +453,11 @@
     OmahaRequestActionTest::SetUp();
     dlc_id_ = "dlc0";
     active_key_ = PrefsInterface::CreateSubKey(
-        kDlcPrefsSubDir, dlc_id_, kPrefsPingActive);
+        {kDlcPrefsSubDir, dlc_id_, kPrefsPingActive});
     last_active_key_ = PrefsInterface::CreateSubKey(
-        kDlcPrefsSubDir, dlc_id_, kPrefsPingLastActive);
+        {kDlcPrefsSubDir, dlc_id_, kPrefsPingLastActive});
     last_rollcall_key_ = PrefsInterface::CreateSubKey(
-        kDlcPrefsSubDir, dlc_id_, kPrefsPingLastRollcall);
+        {kDlcPrefsSubDir, dlc_id_, kPrefsPingLastRollcall});
 
     tuc_params_.http_response =
         "<?xml version=\"1.0\" encoding=\"UTF-8\"?><response "