ANDROID: dm-default-key: set dun_bytes more precisely

Make dm-default-key set dun_bytes to only what it actually needs, so
that it can make use of inline crypto hardware in more cases.

Bug: 144046242
Bug: 153512828
Change-Id: I338e6444e71be9c7c16ce70172d14a8e05301023
Signed-off-by: Eric Biggers <ebiggers@google.com>