Don't dlclose plugins or agents during shutdown.

We were dlclosing agents and the JVMTI plugin during shutdown but it
seems that some agents assume that their code will remain loaded even
after the Agent_OnUnload function returns. This caused segfaults
during shutdown in some situations. Since the runtime is shutting down
anyway there is not much to lose by just not unloading these agents
and the plugins they depend on.

Test: stress --cpu 60
Test: ./art/tools/run-prebuilt-libjdwp-tests.sh \
        --debug \
        --test org.apache.harmony.jpda.tests.jdwp.Events.CombinedEventsTest#testCombinedEvents_05

Bug: 67497270
Bug: 67855829

Change-Id: Ib988c0d21bd12d40f33d709e633312eb68021b38
4 files changed