Have each Display be associated with a DisplayGroup and DPC

Previously the behavior was for each Display to be associated with a
DisplayGroup. This CL restores that behavior.

Additionally, this CL restores that each Display should have its own
DisplayPowerController as this is the mechanism by which each display is
updated.

Fixes: 183403444
Test: atest MultiDisplayKeyguardTests#testUnlockScreen_decoredSystemDisplayChanged_dismissesKeyguardOnUnlock
Change-Id: I31515822d3412948458f8da5c1e89f92f35e495d