Updates to `bp_use_wp_admin_bar()`:

* Default to true
* Use `BP_USE_WP_ADMIN_BAR` constant as true/false.
* Use `bp_force_buddybar()` rather than accessing the option directly.

See #5390.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8579 cdf35c40-ae34-48e0-9cc9-0c9da1808c22