Port update_engine_client ResetStatus command to lib

This is the first step in getting update_engine_client completely off of
direct DBus calls

Test: Ran --reset_status command and verified update_engine received the
      signal.
Bug: 26233663
Signed-off-by: Casey Dahlin <sadmac@google.com>

Change-Id: Iac54d890fd796f91a7813755d6061cf468255347
diff --git a/client_library/client_impl.cc b/client_library/client_impl.cc
index 84ca184..3dba873 100644
--- a/client_library/client_impl.cc
+++ b/client_library/client_impl.cc
@@ -66,6 +66,10 @@
   return StringToUpdateStatus(status_as_string, out_update_status);
 }
 
+bool UpdateEngineClientImpl::ResetStatus() {
+  return proxy_->ResetStatus(nullptr);
+}
+
 bool UpdateEngineClientImpl::SetTargetChannel(const string& in_target_channel) {
   return proxy_->SetChannel(
       in_target_channel,