Avoid throwing exceptions when sessions are closed

[ Merge of http://go/wvgerrit/14404 ]

Closing a non-existant (already closed) session results in an exception
being thrown though mediaDrm. The mediaDrm plugin will now ignore
errors when called though MediaDrm.closeSession.

b/21122725

Change-Id: Ib1c48ade4bdb131195ee56ce2af32bf0cd2166da

libwvdrmengine/mediadrm/src/WVDrmPlugin.cpp

@@ -139,9 +139,9 @@ status_t WVDrmPlugin::openSession(Vector<uint8_t>& sessionId) {
 
 status_t WVDrmPlugin::closeSession(const Vector<uint8_t>& sessionId) {
   CdmSessionId cdmSessionId(sessionId.begin(), sessionId.end());
-  CdmResponseType res = mCDM->CloseSession(cdmSessionId);
+  mCDM->CloseSession(cdmSessionId);
   mCryptoSessions.erase(cdmSessionId);
-  return mapAndNotifyOfCdmResponseType(sessionId, res);
+  return android::OK;
 }
 
 status_t WVDrmPlugin::getKeyRequest(