Fix legacy role holder resolution for assistant, dialer and SMS.

The legacy settings for them can actually remain null and it was
handled specially to use the system default instead. If the legacy
role holders aren't resolved correctly, role implementation will do a
fallback grant which overrides permission state, whereas if they are
resolved correctly the normal fix up won't do an override.

Fixes: 139752137
Test: manual
Change-Id: I5cc301c3b7b459bdfaece0117bcacd984d1cb78f