Clear unused pointer field when sending across binder

Bug: 28377502
Change-Id: Iad5ebfb0a9ef89f09755bb332579dbd3534f9c98

MetadataRetrieverClient::~MetadataRetrieverClient()
{
    ALOGV("MetadataRetrieverClient destructor");
    disconnect();
}

MetadataRetrieverClient::~MetadataRetrieverClient()
{
    ALOGV("MetadataRetrieverClient destructor");
    disconnect();
}
