(1) Change wv_cas_ecm to allow 16 bytes of content_iv

(2) Remove "wrapping_iv" parameters from wv_cas_ecm
(3) Internally derive "wrapping_iv"s and "key_id"s
(4) Add an example binary for demo the usage of wv_cas_ecm

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=218209010

example/wv_cas_ecm_example.cc

@@ -0,0 +1,55 @@
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2018 Google LLC.
+//
+// This software is licensed under the terms defined in the Widevine Master
+// License Agreement. For a copy of this agreement, please contact
+// widevine-licensing@google.com.
+////////////////////////////////////////////////////////////////////////////////
+
+// Example of how to use the wv_cas_ecm library.
+
+#include <string>
+
+#include "gflags/gflags.h"
+#include "glog/logging.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/str_cat.h"
+#include "util/status.h"
+#include "media_cas_packager_sdk/public/wv_cas_ecm.h"
+
+const char kEvenKey[] = "even_content_key";                         // 16 bytes
+const char kEvenContentIv8Bytes[] = "evencont";                     // 8 bytes
+const char kOddKey[] = "odd_content_key.";                          // 16 bytes
+const char kOddContentIv8Bytes[] = "oddcont.";                      // 8 bytes
+const char kEntitlementKeyId[] = "ent_key_id......";                // 16 bytes
+const char kEntitlementKey[] = "entitlement_key.................";  // 32 bytes
+
+int main(int argc, char **argv) {
+  gflags::ParseCommandLineFlags(&argc, &argv, true);
+
+  widevine::cas::WvCasEcm wv_cas_ecm;
+  auto status = wv_cas_ecm.Initialize(/* content_iv_size= */ 16,
+                                      /* key_rotation_enabled= */ true,
+                                      /* crypto_mode= */ 1);
+  if (!status.ok()) {
+    LOG(FATAL) << "Failed to initialize WV CAS ECM, error: " << status;
+  }
+  std::string ecm;
+  status = wv_cas_ecm.GenerateEcm(
+      /* even_key= */ kEvenKey,
+      /* even_content_iv= */
+      absl::StrCat(kEvenContentIv8Bytes, kEvenContentIv8Bytes),
+      /* odd_key= */ kOddKey,
+      /* odd_content_iv= */
+      absl::StrCat(kOddContentIv8Bytes, kOddContentIv8Bytes),
+      /* entitlement_key_id= */ kEntitlementKeyId,
+      /* entitlement_key= */ kEntitlementKey, &ecm);
+  if (!status.ok()) {
+    LOG(FATAL) << "Failed to generate WV CAS ECM, error: " << status;
+  } else {
+    LOG(INFO) << "Generated WV CAS ECM (in hex): "
+              << absl::BytesToHexString(ecm);
+  }
+
+  return 0;
+}