Add config flag in Settings to control whether restricted profiles are offered.

This adds a config resource to specify whether restricted profiles
should be offered as an option when a new user is added. This replaces
the previous check if a device is voice capable, and will be defaulted
to false.

Bug: 202854971
Test: make RunSettingsRoboTests -j40 ROBOTEST_FILTER="com.android.settings.users.UserCapabilitiesTest"
Change-Id: If090fe8d902d6521acfde8c26e801aa4fc4f5ff4