RFR: 8157175: GetNanoTimeAdjustment.java fails with excessive adjustment error

[David Holmes]

bug: https://bugs.openjdk.java.net/browse/JDK-8157175

webrev: http://cr.openjdk.java.net/~dholmes/8157175/webrev/

This essentially backs out the change for

https://bugs.openjdk.java.net/browse/JDK-8154710

which added the use of a faster in-memory timestamp for os::javaTimeMillis.

Unfortunately I did not realize the strong connection between 
os::javaTimeMillis and the java.time related os::javaTimeSystemUTC. 
These two functions must use the same time source so that their 
timestamps are consistent with each other (ie monotonic non-drecreasing).

The changes are a result of "hg backout" but keeping the clean up of 
rmeoving the unused getTimeMillis() and some comment changes. I also 
added a new comment to document the constraint.

Thanks,
David