[Connection Manager] Add LeAcceptlistCallbacks

The connection manager needs to snoop on connection events from GD, so
it will register some callbacks. Since we live in Rust, we don't need a
handler, as we will re-post all events into Rust anyway.

Bug: 272572974
Test: unit
Change-Id: I853c897f18b1cdcde12fec706ab6fc6b8c3e410f