Add fallback ID checking for DisplayDeviceConfig

Some devices already have DisplayDeviceConfig files identified without a stable flag, and new ones will have it with the flag.
This CL tries to find a DisplayDeviceConfig with identifiers in the following priority order:
* physicalDisplayId (new format includes "stable" flag) display_id_<id>.xml
* physicalDisplayId without a stable flag (old system) display_<id>.xml
* portId display_port_<port>.xml

Add dumpsys & toString()

Bug: 166473251
Test: manual & check dumpsys
Change-Id: I9b762b040a16a99f51fb6d7538da4ca8afd9a859