Add GetActiveBootSlot API to boot control interface

This can be used to verify OtaStatus. If on disk pref indicates that we
switched slot and system did not reboot, check with boot control.

Test: th
Change-Id: I416d81a3d68d8c6a204a2c28df6c567ff18d0e49
diff --git a/common/fake_boot_control.h b/common/fake_boot_control.h
index 79e2139..8a68501 100644
--- a/common/fake_boot_control.h
+++ b/common/fake_boot_control.h
@@ -83,6 +83,7 @@
   }
 
   bool SetActiveBootSlot(Slot slot) override { return true; }
+  Slot GetActiveBootSlot() override { return kInvalidSlot; }
 
   bool MarkBootSuccessfulAsync(base::Callback<void(bool)> callback) override {
     // We run the callback directly from here to avoid having to setup a message