shill: Implement write-only properties

Certain properties (e.g. WIFI Passphrase) are write only and must
not be returned when Service.GetProperties() is called over D-Bus.

This CL implements WriteOnlyProperties, a write-only analog of the
read-only ConstProperties.

Also add a ReadablePropertyConstIterator which only returns the
readable properties. Switch over DBus adaptor and PropertyStore
to use that.

BUG=chromium-os:21196
TEST=Added 2 new unittests.

Change-Id: I52815cc395650e0b49e1acac8d4954deeebcee5d
Reviewed-on: https://gerrit.chromium.org/gerrit/11402
Commit-Ready: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Tested-by: Gaurav Shah <gauravsh@chromium.org>
diff --git a/device.cc b/device.cc
index 056b827..e389056 100644
--- a/device.cc
+++ b/device.cc
@@ -245,7 +245,7 @@
 
 void Device::HelpRegisterDerivedStrings(
     const string &name,
-    Strings(Device::*get)(void),
+    Strings(Device::*get)(Error *),
     void(Device::*set)(const Strings&, Error *)) {
   store_.RegisterDerivedStrings(
       name,
@@ -307,7 +307,7 @@
   return StringPrintf("%s:%s", suffix.c_str(), ipconfig_->type().c_str());
 }
 
-vector<string> Device::AvailableIPConfigs() {
+vector<string> Device::AvailableIPConfigs(Error */*error*/) {
   string id = (ipconfig_.get() ? ipconfig_->GetRpcIdentifier() : string());
   return vector<string>(1, id);
 }