Implement OEMCrypto_GetNumberOfOpenSessions

Merged from Widevine CDM repo:
https://widevine-internal-review.googlesource.com/#/c/13560/

Change-Id: Ie19bf3e57d0c4b1621b95bd5912a751ccfbaaa7b

libwvdrmengine/oemcrypto/mock/src/oemcrypto_mock.cpp

@@ -1024,6 +1024,16 @@ bool OEMCrypto_SupportsUsageTable() {
   return supports_usage;
 }
 
+extern "C"
+OEMCryptoResult OEMCrypto_GetNumberOfOpenSessions(size_t* count) {
+  if (LogCategoryEnabled(kLoggingTraceOEMCryptoCalls)) {
+    LOGI("-- OEMCryptoResult OEMCrypto_GetNumberOfOpenSessions(%p)\n", count);
+  }
+  if (count == NULL) return OEMCrypto_ERROR_UNKNOWN_FAILURE;
+  *count = crypto_engine->GetNumberOfOpenSessions();
+  return OEMCrypto_SUCCESS;
+}
+
 extern "C"
 OEMCryptoResult OEMCrypto_GetMaxNumberOfSessions(size_t* maximum) {
   if (LogCategoryEnabled(kLoggingTraceOEMCryptoCalls)) {