Removing unknown sources from security settings

The secure setting is deprecated starting from O. Removing it from the
settings UI so the user cannot change it. The unknown sources can be
managed at a more granular level by starting activity for intent
MANAGE_EXTERNAL_SOURCES

Test: Manually tested that the preference is removed and does not
disturb the other settings on the page.

Bug: 33947615

Change-Id: I8dc559d0e6aaaf5bad78fb6e04469d9ab647411d