Don't send machine and user ID to Omaha anymore. Send a/r pings instead.

This avoids sending a unique ID in order to track active user counts.
Note that this CL doesn't remove the machine/user/Omaha ID/file from
the params object -- it just makes them unused/obsolete. Removal will
be done in a subsequent CL in an effort to make this CL smaller.

BUG=1439
TEST=unit tests, x86-generic, arm-generic, gmerged and inspected logs

Review URL: http://codereview.chromium.org/2856070
diff --git a/prefs.cc b/prefs.cc
index 38129c9..39f5908 100644
--- a/prefs.cc
+++ b/prefs.cc
@@ -13,6 +13,9 @@
 
 namespace chromeos_update_engine {
 
+const char kPrefsLastActivePingDay[] = "last-active-ping-day";
+const char kPrefsLastRollCallPingDay[] = "last-roll-call-ping-day";
+
 bool Prefs::Init(const FilePath& prefs_dir) {
   prefs_dir_ = prefs_dir;
   return true;