AccountUnlockScreen: Explicitly register themeChange receiver

Make sure that we explicitly register the receiver and don't attempt to
unregister otherwise.

Replicated from commit : f7ca5108

To reproduce crash :

- Use a secure lockscreen
- Do failed attempts till timeout
- Click account unlock, then cycle screen power

Change-Id: Idbfb7969b657086e31600ee2cb52b9d1f42bf70a