Add SystemUI component to watch for keyboard attachment.

Add a new SystemUI component to watch for keyboard attachment /
detachment. If the config specifies the name of a keyboard that is
packaged with the device, then SystemUI will ask the user if they
would like to enable BT (if disabled) and then attempt to pair to the
device.

Bug: 22876536
Change-Id: I786db35524d49706d5e61d8b8bc71194d50113f3