Allows sharing of keys between sessions

This change allows the app to specify that keys may be shared by sessions.
The app enables this by setting the session sharing properties in DRM Plugin.
Keys are shared only amoungst the specified group of sessions.

Merged from widevine CDM repo
 * https://widevine-internal-review.googlesource.com/#/c/8019/
 * https://widevine-internal-review.googlesource.com/#/c/8021/

Bug: 11013707
Change-Id: I52db41a53138b4fc563ebc6d38a623f23f7cdfb5

libwvdrmengine/cdm/core/src/cdm_engine.cpp

@@ -525,6 +525,29 @@ bool CdmEngine::IsKeyValid(const KeyId& key_id) {
   return false;
 }
 
+bool CdmEngine::FindSessionForKey(
+    const KeyId& key_id,
+    CdmSessionId* session_id) {
+  if (NULL == session_id) {
+    LOGE("CdmEngine::FindSessionForKey: session id not provided");
+    return false;
+  }
+
+  uint32_t session_sharing_id = Properties::GetSessionSharingId(*session_id);
+
+  for (CdmSessionMap::iterator iter = sessions_.begin();
+       iter != sessions_.end(); ++iter) {
+    CdmSessionId id = iter->second->session_id();
+    if (Properties::GetSessionSharingId(id) == session_sharing_id) {
+      if (iter->second->IsKeyValid(key_id)) {
+        *session_id = id;
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
 bool CdmEngine::AttachEventListener(
     const CdmSessionId& session_id,
     WvCdmEventListener* listener) {