Remove unused DEVICE_POWER permission

Looking through the checkin history, it appears that DEVICE_POWER was
once a permission required to set Window parameters that would dim the
screen. See ag/33765 for details. This technique is no longer used in
code, and the permission is no longer required.

Bug: 20273223
Change-Id: I83fbee19689ef3095f95540e606abbf5e9081ab0