Bluetooth: Fix central from failing to start encryption

This fixes a regression introduced in commit 6af5d1cd
("Bluetooth: Compress bt_keys struct").

Instead of passing a value zero as the random number, the
value at the RAM address zero was being used by the start
encryption function call. It is now fixed by consistently
using byte-array to store EDiv and Rand values.

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