Remove the distinction between key and device descriptors

Change-Id: I12ac0f77124caae0be7b3a178683e5a3213f9beb
Reviewed-on: https://skia-review.googlesource.com/c/189869
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>