Tweak ExpiringCache closer to upstream OpenJDK8u121-b13 code.

This is a refactoring only, without behavior change.

Also, document that/why this code is different from upstream.
The difference is due to what looks like a slight difference
in the compiler (javac vs. jack).

Bug: 35910877
Test: Treehugger succeeds.
Change-Id: Ic6c036aea50d9d499bd68c18599c3a21b346696a
1 file changed