Support LE Subrating

[khsiao-google]

Handle the following HCI commands / events and make device API for them

1. HCI_LE_Subrate_Change
2. HCI_LE_Set_Default_Subrate
3. HCI_LE_Subrate_Request

[barbibulle]

The way I understand the spec, I think that when there's a LE Connection Update Complete event, if the subrate factor and continuation number are only reset if there's a change in the connection interval. So if there's no change, the Connection.Parameters object that's created in the on_connection_parameters_update method should reuse the existing values for subrate_factor and continuation_number.

[khsiao-google]

The way I understand the spec, I think that when there's a LE Connection Update Complete event, if the subrate factor and continuation number are only reset if there's a change in the connection interval. So if there's no change, the Connection.Parameters object that's created in the on_connection_parameters_update method should reuse the existing values for subrate_factor and continuation_number.

I think that's a great point. Already modify the on_connection_parameters_update so that it only resets when connection interval changes.