Revert "Disable 160-read-barrier-stress temporarily"

And reduce the number of fields in class ManyFields from
10000 to 5000 to reduce memory pressure on Jack. Split
the ManyFields class in a simple hierarchy to keep files
under 64KiB and allow compilation with javac.

Test: testrunner.py --host -j 48
Test: run-test --host --jvm 160-read-barrier-stress
Bug: 37335480

This reverts commit 10107499.

Change-Id: I52217fd2c9d00a5b045a7faa6e4dc1438c7f93b1