This is a copy of the Widevine CL:
https://widevine-internal-review.googlesource.com/#/c/9708/
This CL refactors some of code in oemcrypto/mock and oemcrypto/test in
preparation for adding usage table code.
Change-Id: I7e58c8ecd6d92b3e177cb915733212fcad645485