Replace BT_OCTET16 with Octet16

BT_OCTET16 was a uint8_t[16], that is a pointer.
This means that it was not copyable, and one had to be always aware of
what this type really means when passing it to function.

Octet16 is std::array<uint8_t, 16> and is copyable. It can also be
copied using "=" operator, instead of memcpy, or ARRAY_TO_STREAM.

Octet16 can also be returned from function by value. Thanks to it, some
smp functions can be simplified, by returning value instead of accepting
pointer to output.

Test: net_stack_smp_test
Change-Id: I0687a948e5807f76ec3d1b5d1d4d2ad2b50b87dd