Expand DISALLOW_GRANT_ADMIN restriction

Expand the functionality of DISALLOW_GRANT_ADMIN to include the following:

If applied to a user without admin privileges, it functions as before (prevents them from gaining admin rights).
If applied to a user with existing admin privileges, it additionally won't let the user give admin access to other users.

Test: only java doc change
Bug: 341695742
Flags: only java doc change
Change-Id: I33b7000690af45407f3f52d41132da92dacdedba