[Private GATT] Clear services when server closed

If we just close the server, we keep an Rc<> to the live server with its
attributes. We should instead clear all the services first (and
eventually send a srvc_chg indication) before closing.

Test: unit
Bug: 255880936
Change-Id: I2d8430b57cd8031bb41f19c1ddb2cabcc9e24fbe