(This is a merge of http://go/wvgerrit/17661)
Because Widevine core always has a decrypt mode set, even on unencrypted
calls where we potentially do not know the crypto mode, it will reject
unencrypted decrypt calls for sessions that have been flagged as
AES-CBC, since the crypto mode defaults to AES-CTR.
The fix is to not validate the crypto mode on unencrypted decrypt calls,
as the data won't be getting decrypted anyway.
Bug: 28423928
Change-Id: If848834dd498ca96983a2b69d448b8d81d50e0a4
6d2822f6c6