Disallow changing backup activation state for private profile

Private profile is used for the 'private space' feature and backup is always disabled for that user. Not even the system server or root user can enable backup for it.

Test: atest BackupManagerServiceTest
Flag: EXEMPT bugfix
Bug: 404533205
Change-Id: I8235f1a0dbdb23d9e7ce3d40b624f9d3e85ce70f