Restrict admin status change when DISALLOW_GRANT_ADMIN is present

This change addresses a security gap where users with the "DISALLOW_GRANT_ADMIN" restriction could still grant admin privileges to others, potentially undermining supervised user models.  We've extended the existing logic to ensure that restricted users cannot grant or receive admin privileges when the DISALLOW_GRANT_ADMIN restriction is in place. This prevents scenarios where supervised users could create new admins to bypass restrictions and factory reset the device.

In addition to the core functionality improvement, significant code refactoring has been done to untangle complex multiple if conditions. The logic for determining when admin status changes are allowed is now clearer and more readable, taking into account both the "current user" (initiating the change) and the "target user" (whose privileges are being modified).

Bug: 357056776
Test: atest UserDetailsSettingsTest and manually verified the user having DISALLOW_GRANT_ADMIN restriction is not allowed to change admin status of any other user.
Flag: android.multiuser.unicorn_mode_refactoring_for_hsum_read_only
Change-Id: If02c9355ee7ce285b5b7bcddae630d716d5bf333