Treat the (0,0) Pattern as 'cbcs'

(This is a merge of http://go/wvgerrit/94928.)

In OEMCrypto v16, we dropped support for 'cens' and 'cbc1'. However, we
did not redefine the pattern (0,0) to be a valid pattern for 'cbcs', even
though it was no longer being used to signal 'cbc1'. Instead, we made
the CDM reject CTR with a pattern ('cens') and CBC with a (0,0) pattern
('cbc1') to mirror the behavior of OEMCrypto v16.

However, some apps have been using 'cbc1' mode to decrypt audio in
'cbcs' content. This is normally not possible but is possible for a
subset of content. Furthermore, it is easy to do by accident because of
the way most packagers package 'cbcs' audio and the special significance
Widevine has historically given the (0,0) pattern.

This patch updates the CDM to not reject CBC with a (0,0) pattern but
instead treat it as 'cbcs' content. To decrypt it correctly, the pattern
is treated specially inside the CDM core and converted to the
recommended equivalent pattern — (10,0) — before passing the content to
OEMCrypto.

For more specifics, please see the design doc: http://go/vclfg

Bug: 150219982
Test: ExoPlayer Demo App 'cbcs' Content
Test: GTS 'cbcs' Content
Change-Id: I334ff15db5f7b7d62040a036ba6d17515c3caee4

libwvdrmengine/cdm/core/src/crypto_session.cpp

@@ -1468,6 +1468,13 @@ CdmResponseType CryptoSession::Decrypt(
   // Convert the pattern descriptor
   OEMCrypto_CENCEncryptPatternDesc oec_pattern{params.pattern.encrypt_blocks,
                                                params.pattern.skip_blocks};
+  // TODO(b/146581957): Remove this workaround once OEMCrypto treats (0,0) as
+  // 'cbcs' instead of 'cbc1'.
+  if (params.cipher_mode == kCipherModeCbc && oec_pattern.encrypt == 0 &&
+      oec_pattern.skip == 0) {
+    // (10, 0) is the preferred pattern for decrypting every block in 'cbcs'
+    oec_pattern.encrypt = 10;
+  }
 
   // Check if a key needs to be selected
   if (is_any_sample_protected) {