Bluetooth: controller: Use single PPI channel for AA capture

Earlier design captured AA twice in the first Rx in a slave
connection event and retained one of the capture until end
of event to calculate drift.

Design updated to use single capture of AA and save the
first AA capture in a slave connection event in RAM instead.

This frees up a PPI channel in the controller design.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>