samples: bt: reduce system heap memory for the hci_ipc sample

Moving from TinyCrypt to PSA Crypto API caused failures in the cpu_net
build due to RAM being overflowed. It turned out that 8192 bytes were
allocated for system heap memory, but Mbed TLS is the only user
of that memory (I found this though puncover) for AES purposes.
We reduce that to 1024 bytes because this should be enough for
this purpose.

Note: albeit this is also a standalone example, it's used extensively
in other samples/tests and babblesim, so a failure in building it
propagates in a lot of other failures.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>