Bluetooth: samples: Fix minimal configuration stack sizes

With recent changes to the host stack and crypto subsystem, the actual
usage of the system and long workqueues has changed.

Here's the measured usage when doing LE SC pairing on a Cortex M0 target:

BT LW WQ            : STACK: unused 360 usage 1040 / 1400 (74 %); CPU: 4 %
sysworkq            : STACK: unused 0 usage 1024 / 1024 (100 %); CPU: 0 %

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>