Cleanup dock battery support

Send an intent as soon as the device is undocked to get a timely update
of the SystemUI.

Adds a proper implementation of dock detection in BatteryService via
"dock_battery/device/ec_dock". This will probably have to be rewritten
in a more generic way to support other manufacturers as well (once we
support other docks at all).

Change-Id: Ib1088c118c8695fd93ba085d976ca1eb374ae71e