Merge from Widevine repo of http://go/wvgerrit/77609
For v15.2 we require that nonces not collide across sessions and there are
restrictions placed on the mac key's IV in LoadKeys.
Test: ran unit tests on reference code
Bug: 131325434
Bug: 131326334
Change-Id: I1bb01c30d8c15d66d762c28b57d7700c44daa835