Make the initial value of default_visibility "public" in the BP_XProfile_Field class

This fixes a graphical bug where on the "Add Field" screen, none of the
"Default Visibilty" radio buttons were selected by default.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7509 cdf35c40-ae34-48e0-9cc9-0c9da1808c22