subsys/settings: Update bluetooth module

Updated the bluetooth module to use static handlers. Removed the
old bt specific static registration.

The routine bt_settings_init() is still calling settings_init() which
IMO is not needed anymore.

Updates:

changed SETTINGS_REGISTER_STATIC() to SETTINGS_STATIC_HANDLER_DEFINE()
changed settings_handler_stat type to settings_handler_static type
removed NULL declarations
renamed bt_handler to bt_settingshandler, as bt_handler already exists.
renamed all bt_XXX_handler to bt_xxx_settingshandler to avoid any
overlap.
changed SETTINGS_STATIC_HANDLER_DEFINE() to create variable names from
_hname by just prepending them with settings_handler_.
updated all bt_xxx_settings_handler to just bt_xxx.

Signed-off-by: Laczen JMS <laczenjms@gmail.com>