Introduce system UI immersive confirmation dialog feature flag

To ensure a smooth transition for the non trivial refactor to move the
immersive confirmation dialog implementation from the system server to
the system UI, introduce a new feature flag for that. Also, replace the
existing control flag with the feature flag.

Bug: 359713629
Flag: com.android.window.flags.system_ui_immersive_confirmation_dialog
Test: build
Change-Id: Iddc81abe27163e888b845fd979b48c0f918d8e33