[ Merge of http://go/wvgerrit/194571 ] Default to OEMCrypto_CipherMode_CBC instead of OEMCrypto_CipherMode_CENC which is not used by CAS. Test: CAS unit tests Bug: 325639114 Bug: 322928572 Merged from https://widevine-internal-review.googlesource.com/193650 Change-Id: I8876d5262643015fb6a322eae6444ef4001d146d