Add setting constant to mirror built-in display

Add a new secure setting contant, which is used to mirror built-in
display in all the connected displays.

Bug: 352461502
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Test: adb shell settings put secure mirror_built_in_display <1|0>

Change-Id: Ia7be29a2021cf8d5e565d5d0bfe3a51635ec5e53