vold: downgrade lookup_key_ref() failure message to DEBUG level

Now that lockUserKey() will be called for every user each time the
framework is started, it's a normal case that the key being removed
doesn't exist.  So downgrade the log message about being unable to find
a key from ERROR to DEBUG.

Test: build and boot, message is now debug level.
Change-Id: I2d1dce8dd8825b02982eab1c902aa10fe5b54b84
diff --git a/FsCrypt.cpp b/FsCrypt.cpp
index ea8bb73..b7d3928 100644
--- a/FsCrypt.cpp
+++ b/FsCrypt.cpp
@@ -269,7 +269,7 @@
                            std::string* raw_ref) {
     auto refi = key_map.find(user_id);
     if (refi == key_map.end()) {
-        LOG(ERROR) << "Cannot find key for " << user_id;
+        LOG(DEBUG) << "Cannot find key for " << user_id;
         return false;
     }
     *raw_ref = refi->second;