DockBatteryMeter: Avoid double detach when view is rebuilt

If the view is reconstructed for some reason (like applying a theme)
on a device without a dock battery, it'll try to detach and unregister
twice. The second unregister will case an NPE since the first one
killed the listener. Fixit.

Change-Id: Idcc0c7c87ef205ddc085a9e036fe630afd9d7012