Add flag info to displays dump output proto

FLAG_PRIVATE information is needed for cts on physical private display

Bug: 133794475
Test: -Build and flash (make -j framework; aae flash 1s)
- Add "Log.d(TAG, display.toString())" in code to print out detailed
information about display. (Note: Display class is imported from
android.server.wm.WindowManagerState.Display)
- Check "mFlags=.." is printed

Change-Id: I6481dff51b140d3a9320e1e92b06fff531ba34e9