Add feature flag for Emergency Callback Mode Notification

This commit adds feature flag for Emergency Callback Mode Notification.
This flag allows the use to notify the emergency callback mode for
call/SMS to other applications.

Bug: 359064059
Test: build pass
Flag: com.android.internal.telephony.flags.emergency_callback_mode_notification

Change-Id: I36c28091a88973dea83d2f9d1d4943c06e2a8ca1