Only create DPCs for LocalDisplays & FLAG_OWN_DISPLAY_GROUP

Instead of all Displays having their own DisplayPowerController, this CL
makes it so that only LocalDisplays or displays which have explicitly
requested a DisplayGroup will have a DPC created.

This preserves existing behavior for non-physical displays.

Bug: 181901142
Test: launch Android Auto in DHU mode and turn off main display
Change-Id: I9aa333f8dce05b5dcebf11d16fd8b0f61c3377cd