Hook in at after_setup_theme instead of plugins_loaded

When used with bbPress, enabling debug mode shows this error message: "Notice: bbp_setup_current_user was called incorrectly. The current user is being initialized without using $wp->init(). Please see Debugging in WordPress for more information. (This message was added in version 2.3.) "

Changing the hook seems to maintain the desired functionality, w/o the error message. See also https://core.trac.wordpress.org/ticket/24169