ART: Ensure dex caches keep classloader live

Live dex caches must keep their associated classloader live. Otherwise
the classloader may get unloaded, attempting to free DexFiles which
cannot be unregistered.

Test: art/test/testrunner/testrunner.py -b --host
Test: m test-art-host-gtest
Bug: 149098478

(cherry-picked from commit 8a1a0f719e42746840f88aad079711e9d951b797)

Merged-In: I0eed5b3b46ed681c739d6923a57d0878afbba1a7
Change-Id: I0eed5b3b46ed681c739d6923a57d0878afbba1a7