Always instantiate DisplayAddress.Physical

In legacy multi-display mode with at most two physical displays,
physical display IDs reduce to the port IDs for the internal and
external connections.

This CL instantiates a DisplayAddress.Physical whose getModel method
returns null, such that port-dependent logic (e.g. display settings
and input association) works on legacy systems without EDID support.

Bug: 130672828
Test: dumpsys display
Change-Id: I12a40078c5e3666e59283623ec600e8d76493359