Move ParseConnection*() to connection_utils.

We don't need real_shill_provider if USE_DBUS is 0, but we still need
these functions, and they have a duplicate copy in connection_manager,
so put them in utils and deduplicate.

Also moved StringForConnectionType() to connection_utils.

Bug: 28800946
Test: mma

Change-Id: If535fdc52bc8fb267921bea02b31d8d6580d5d54
diff --git a/update_manager/boxed_value_unittest.cc b/update_manager/boxed_value_unittest.cc
index 47bfd8f..3d0c72e 100644
--- a/update_manager/boxed_value_unittest.cc
+++ b/update_manager/boxed_value_unittest.cc
@@ -31,6 +31,8 @@
 
 using base::Time;
 using base::TimeDelta;
+using chromeos_update_engine::ConnectionTethering;
+using chromeos_update_engine::ConnectionType;
 using std::list;
 using std::map;
 using std::set;