Fix app_fd and mtu for offload RFCOMM socket

In on_srv_rfc_connect_offload it should not assign INVALID_FD to app_fd, which would block the socket from opening.
Instead it assigns INVALID_FD to app_fd in on_btsocket_rfc_opened_complete after sending the app connect signal.

Updated the selection of local mtu for offload socket by skipping the guardrail of L2CAP_MTU_SIZE in non-offload stack.

Also removed several redundant function calls in on_srv_rfc_connect_offload and on_btsocket_rfc_opened_complete.

Flag: com.android.bluetooth.flags.socket_settings_api
Bug: 342012881
Bug: 374358112
Test: m com.android.btservices && OEM device tests
Change-Id: Ie71ee75c3e05f8531f11b6b86871d0be2474db17