SF,HDR: Add readonly flag for connected display HDR feature

Bug: 374182788
Bug: 374187407
Test: adb shell dumpsys SurfaceFlinger | grep connected_display_hdr
Flag: com.android.graphics.surfaceflinger.flags.connected_display_hdr
Change-Id: I29ba7ecb289f55e6d2be2b74f82bf012d3530ec5
Signed-off-by: Sasha McIntosh <sashamcintosh@google.com>