Add guardrail for offload RFCOMM socket local mtu

The offload stack may not know about the L2CAP mtu used for the offload RFCOMM socket, so it is safer to gate the local mtu in the BT stack in case a very large mtu is requested for the socket.

Flag: com.android.bluetooth.flags.socket_settings_api
Bug: 342012881
Bug: 374358112
Test: m com.android.btservices
Change-Id: I18e5de7514cde1c67c9c7ace201cd2a18dd264e9