Switch to using shared_ptr for Session Metrics

[ Merge from http://go/wvgerrit/71443 ]

The assumption that the metrics will always outlive the CdmSession
instance appears not to always hold (at least in a non-android
multi-threaded solution). The shared_ptr ensures that the metrics
are available even in these rare race conditions.

BUG: http://b/123321465
Test: CDM unit tests. Also http://go/wvgerrit/71264 parallel tests.
Change-Id: Iaa6a8f6c0fdc46a911789759d6e1228d849aa237

libwvdrmengine/cdm/metrics/src/metrics_collections.cpp

@@ -154,7 +154,7 @@ void CryptoMetrics::Serialize(WvCdmMetrics::CryptoMetrics *crypto_metrics)
             crypto_metrics->mutable_oemcrypto_update_usage_entry());
 }
 
-SessionMetrics::SessionMetrics() {}
+SessionMetrics::SessionMetrics() : session_id_(""), completed_(false) {}
 
 void SessionMetrics::Serialize(WvCdmMetrics::SessionMetrics *session_metrics)
     const {
@@ -268,10 +268,10 @@ EngineMetrics::~EngineMetrics() {
   }
 }
 
-SessionMetrics *EngineMetrics::AddSession() {
+std::shared_ptr<SessionMetrics> EngineMetrics::AddSession() {
   std::unique_lock<std::mutex> lock(session_metrics_lock_);
   active_session_metrics_list_.push_back(std::make_shared<SessionMetrics>());
-  return active_session_metrics_list_.back().get();
+  return active_session_metrics_list_.back();
 }
 
 void EngineMetrics::RemoveSession(wvcdm::CdmSessionId session_id) {