SF: Populate DisplayConnectionType in DisplayInfo

Without HWC support, fall back to categorizing the primary display as
internal, and secondary displays as external.

Also, validate displays in HWComposer for other 2.4 APIs.

Bug: 134771872
Test: dumpsys SurfaceFlinger --displays
Test: dumpsys display
Test: libsurfaceflinger_unittest
Change-Id: I139eeab9cc34e21a1b8584a2fcb9c9fd5dc43ac3