Keep "None" set for assistant upon device upgrade.

If the assistant role is added for the first time, the device is
upgrading, and legacy role resolution didn't set any holder, it
implies the user selected "None" for assistant, and we need to keep
that in our new implementation.

Fixes: 141255935
Test: manual
Test: Upgrade P => Q without this CL and "None" assistant is lost
Test: Upgrade P => Q with this CL and "None" assistant is kept
Test: Fresh Q with this CL gets the default assistant set
Change-Id: Ie2668d29c43751686dbe9523bcc298b73618fc91