Shim GATT: Use Fixed channel manager to connect

Legacy GATT uses its own connection manager to connect, but in GD it
should be managed by HCI module. Redirect connection request to L2CAP
module (which will be handled by HCI-ACL module).

Also use no-op for existing LE L2CAP public API used by GATT for now.

Tag: #gd-refactor
Bug: 158861440
Test: nRF
Change-Id: Idcdcba6d3aec774fe4df8edab5396728d36acf6b