Update VMRuntime.preloadDexCaches() for hash-based arrays.

Do not try to fill entries that already contain some
resolved item to avoid unnecessary work. We do not know
which of the conflicting items may be needed later.

Test: m test-art-host-gtest
Test: testrunner.py --host
Test: Nexus 6P boot.
Bug: 30627598
Bug: 63892794

(cherry picked from commit 753d6137)

Change-Id: I36ed67d112bf94d0fdd1a879ededb05570da6420