SF: Initialize DisplayDevice with the active config index

This is a port of aosp/580542, with added unit test changes now that the
correct config is being set in the DisplayDevice.

Bug: 69807179
Test: atest libsurfaceflinger_unittest

Change-Id: I51c13678bc28c715fa116ad127478d4afa3d0349
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index d7f3b08..f968948 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -164,6 +164,7 @@
 
     std::shared_ptr<const HWC2::Display::Config>
             getActiveConfig(int32_t displayId) const;
+    int getActiveConfigIndex(int32_t displayId) const;
 
     std::vector<ui::ColorMode> getColorModes(int32_t displayId) const;