Maximize L2CAP socket MTU

Introduce L2CAP_SDU_LENGTH_LE_MAX constant, and use it for LE CoC
sockets, while still keeping the old value for Classic sockets.

This also require change in how we send the rx_mtu up the stack - it
must be read from the socket property.

Additionally in this patch, l2cap_socket.mtu is renamed to tx_mtu to
distinguish it from rx_mtu. Assignment of this variable is also added
when we receive it from the remote.

Bug: 68359837
Test: tranfer file using OPP, do obex, run LE CoC tests. Run tests
between device with this patch, and older android phones.
Change-Id: Iffa20a6c6a93d5787bada7bf28655e9880d8c1a0