Implement Phase 5 of Separate Power States

DisplayPowerRequestManager listens for the addition, removal, and change
of Displays and will appropriately create DisplayPowerRequests to manage
their power states.

At this stage the DisplayPowerRequests are simply created; only the
DisplayPowerRequest associated with the default display is interacted
with.

Test: manual
Bug: 138328918
Change-Id: I03f0c95aa23fc711d8d66150f315eed52bc57b2c