docs: Added note about limited support for hardware key attestation

Most devices running Android N support only software-based key
attestation, so added several notes to the pages describing this
tool to help developers understand how to tell whether a device
supports only software-based key attestation and the associated
effects of this support level.

Bug: 28851641
Change-Id: I16c8ad7702814eba1e4b8b3ee7e9a569d08c3ff4