Add missing "dock" plug type to battery stats

Add the missing dock plug type to match the behavior in
BatteryManager.java

https: //cs.android.com/android/platform/superproject/main/+/main:frameworks/base/services/core/java/com/android/server/BatteryService.java;l=500;

Cherry-pick from https://r.android.com/3390927

Flag: EXEMPT bugfix

Change-Id: I495b39d2c26861d76336a0ca255da059218e9632