Enable class unloading in dex2oat for quicken multidex

Enable class unloading for the quicken compilation filter. This
reduces RAM used by dex2oat when compiling multidex apps with the
quicken filter.

Unit test is already in dex2oat_test.

Bug: 63467744
Test: test-art-host
Test: test/testrunner/testrunner.py --interpreter --host -j40
Change-Id: Icde116fc066aaae9bf0ee951f277a618be60ccf0