Fix signedness of DisplayAddress.Physical ports

Interpret port as unsigned byte for consistency with uint8_t in SF/IF,
and clarify signedness in doc comments.

Bug: 143713778
Test: adb shell dumpsys display | grep physicalPort
Test: LocalDisplayAdapterTest
Test: DisplayWindowSettingsTests
Change-Id: I56ce0067367372a3a747fde783a1956975e833a0