RFR: 8201327: Make Sensor deeply immutably thread safe

[Martin Buchholz]

Another little cleanup to make Google's race detector happy.

8201327: Make Sensor deeply immutably thread safe
http://cr.openjdk.java.net/~martin/webrevs/jdk/Sensor-init/
https://bugs.openjdk.java.net/browse/JDK-8201327