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.

Peak dex2oat PSS according to procrank for a large app:
243247K -> 147706K for quicken filter.

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

(cherry picked from commit 6a2b67be)

Change-Id: I476ee76b7c9efb57238673a117dcb94bbc09f430