Merge from Widevine repo of http://go/wvgerrit/27461
In order to sign a license release message, the mac keys from the
usage entry should be used whenever keys have not been loaded.
This CL updates the reference code, the unit tests, and the level 3
oemcrypto.
b/38203566
Test: unit tests passing on bullhead.
Change-Id: Ic71fee4b4b7b45801548ab80fbbbf8f4ccab3e6e