Drop minimum RKP challenge size to 16 bytes

The current RKP server produces challenges smaller than 32 bytes.
As existing devices in the field may have some length limitations
due to this, let's not start sending larger challenges to those
devices. Instead, drop the challenge to 16 bytes to maintain compat.
There should be plenty of entropy in 16 bytes.

Test: n/a
Change-Id: I1dfd9b4b06131df907683207e4b6bfb2d1c93d65