Add permission for HeadlessSystemUser Camera.

To allow only platform signed apps to access camera
as User 0 in Headless user Mode, add a new permission
android.permission.CAMERA_HEADLESS_SYSTEM_USER with
protection level of "signature".

Bug:296959023
Test: Test that headless system user is not able to access
the camera without the new permission.

Change-Id: I8fab31e1d7e483b476fe1761c8b35e1766119b25