Revised GPIO module interface + GPIO discovery logic

* The GpioHandler class is no longer a static singleton, rather an
  ordinary object with a dynamic guard against multiple instances. This
  makes testing/mocking a lot easier and simplifies implementation.

* It uses a basic, mockable udev interface; the module comes with
  complete unit testing of the discovery mechanism.

* Corresponding changes to user classes, including UpdateAttempter and
  UpdateCheckScheduler.

Note that the implementation of the test mode signaling protocol is
currently a no-op, always returning false, and hence has no effect on
the update process yet. This mechanism will be implemented in a later
CL.

BUG=chromium-os:25397
TEST=Builds and passes unit tests (including new ones)

Change-Id: I2f6254db6799ff5ef8616314890833f6e3269ff6
Reviewed-on: https://gerrit.chromium.org/gerrit/22869
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Ready: Gilad Arnold <garnold@chromium.org>
diff --git a/update_attempter_mock.h b/update_attempter_mock.h
index 50b5c25..bdb83ff 100644
--- a/update_attempter_mock.h
+++ b/update_attempter_mock.h
@@ -15,7 +15,7 @@
 class UpdateAttempterMock : public UpdateAttempter {
  public:
   explicit UpdateAttempterMock(MockDbusGlib* dbus)
-      : UpdateAttempter(NULL, NULL, dbus) {}
+      : UpdateAttempter(NULL, NULL, dbus, NULL) {}
 
   MOCK_METHOD5(Update, void(const std::string& app_version,
                             const std::string& omaha_url,