[Private GATT] Refactor to remove Rc<>

This CL introduces the abstraction of SharedBox<>, which
wraps Rc<> to avoid accidental circular references / leaks.

Without this, we could leak AttServerBearers by e.g. storing
an Rc<> to them in a spawned future, or by actually having a
circular reference.

Bug: 255880936
Test: unit

Change-Id: If60bd51d4ecda799de465d2e54a2976cf4e6793a