update_engine: Use PrefsInterface from SystemState

There is no need to pass the Pref class around (at least not in cros)
since we have the SystemState as the global context and we can get the
pref from there.

BUG=b:171829801
TEST=cros_workon_make --board reef --test update_engine

Change-Id: I9f5fb8a118fab2ef0e188c42f746dafb1094972c
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2548740
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/cros/payload_state.h b/cros/payload_state.h
index ad19074..db54865 100644
--- a/cros/payload_state.h
+++ b/cros/payload_state.h
@@ -483,10 +483,9 @@
   size_t payload_index_ = 0;
 
   // The index of the current URL.  This type is different from the one in the
-  // accessor methods because PrefsInterface supports only int64_t but we want
+  // accessor methods because |PrefsInterface| supports only int64_t but we want
   // to provide a stronger abstraction of uint32_t.  Each update to this value
-  // is persisted so we resume from the same value in case of a process
-  // restart.
+  // is persisted so we resume from the same value in case of a process restart.
   size_t url_index_;
 
   // The count of failures encountered in the current attempt to download using