Don't show Settings as a home app anywhere in UI.

FallbackHome inside Settings should not be explicitly set as the home
activity by user, which will prevent the device from getting a real
home app running. It is already hidden in Default apps settings, but
it needs to be hidden in the request role dialog as well.

Fixes: 139919985
Test: Manually confirm Setting doesn't appear in request role dialog
Test: or Default apps settings.
Change-Id: I6f8e935ebc6ff3aed6a54c595a2e5492321432a8