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/boot_control_stub.h b/common/boot_control_stub.h
index dcddbae..3167115 100644
--- a/common/boot_control_stub.h
+++ b/common/boot_control_stub.h
@@ -56,6 +56,7 @@
   bool IsSlotBootable(BootControlInterface::Slot slot) const override;
   bool MarkSlotUnbootable(BootControlInterface::Slot slot) override;
   bool SetActiveBootSlot(BootControlInterface::Slot slot) override;
+  Slot GetActiveBootSlot() override { return kInvalidSlot; }
   bool MarkBootSuccessfulAsync(base::Callback<void(bool)> callback) override;
   bool IsSlotMarkedSuccessful(BootControlInterface::Slot slot) const override;
   DynamicPartitionControlInterface* GetDynamicPartitionControl() override;