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_check_scheduler_unittest.cc b/update_check_scheduler_unittest.cc
index 098651b..965ce9d 100644
--- a/update_check_scheduler_unittest.cc
+++ b/update_check_scheduler_unittest.cc
@@ -30,7 +30,7 @@
 class UpdateCheckSchedulerUnderTest : public UpdateCheckScheduler {
  public:
   UpdateCheckSchedulerUnderTest(UpdateAttempter* update_attempter)
-      : UpdateCheckScheduler(update_attempter) {}
+      : UpdateCheckScheduler(update_attempter, NULL) {}
 
   MOCK_METHOD2(GTimeoutAddSeconds, guint(guint seconds, GSourceFunc function));
   MOCK_METHOD0(IsBootDeviceRemovable, bool());