LE: Add API for sending connection parameter update

Add Link::SendConnectionParameterUpdate to send LE connection parameter
update.

Next step: Expose this API to channel (or something else). We also need
to send HCI command instead of L2CAP when both supports 4.1.

Test: cert/run --host
Change-Id: I5e1cd28796be4d4b17a189a942fb08abb65cc865