Start mirroring on the external display when lock task mode begins

and update the external display mirroring to ensure it match the system
settings when lock task mode ends.
This CL also ensures the external display shows mirroring if it's
connected to a device in lock task mode.

Bug: 408988940
Test: manaually test using TestDPC
Test: DisplayManagerServiceTest
Flag: com.android.server.display.feature.flags.enable_display_mirror_in_lock_task_mode
Change-Id: Iaafa2bdaf7d300d1e36e7c1e292d5262fce235d5