Move InattentiveSleepWarningController callbacks to PowerUI

The inattentive sleep warnings are related to the power state of the
deivce, and by keeping the inattentive sleep warning callbacks inside
PowerUI, we reduce the number of components that have to be configured
across the different SystemUI packages.

Bug: 137633812
Test: adb shell settings put secure attentive_timeout 32000
      Check overlay appears

Change-Id: I7896c395a26eb5bdd67bb3ab48fc590beef8cec3