Allow testing of alternative JDWP implementations.

This adds support for a jdpa.settings.debuggeeAgentArgument property
that will allow one to change the way the debugee's JDWP agent is
invoked.

Test: art/tools/run-jdwp-tests.sh
Test: vogar --verbose --mode=jvm --timeout 100000 \
        --vm-arg -Djpda.settings.verbose=true \
        --vm-arg -Djpda.settings.timeout=60000 \
        --vm-arg -Djpda.settings.waitingTime=60000 \
        --vm-arg -Djpda.settings.transportAddress=127.0.0.1:55107 \
        --vm-arg -Djpda.settings.debuggeeJavaPath=/usr/bin/java \
        --vm-arg -Djdpa.settings.jdpa.settings.debuggeeAgentArgument="-agentpath:" \
        --vm-arg -Djdpa.settings.debuggeeAgentName=/usr/lib/jvm/java-8-openjdk-amd64/jre/lib/amd64/libjdwp.so \
        --toolchain jdk --language CUR \
        --classpath ${ANDROID_HOST_OUT}/../common/obj/JAVA_LIBRARIES/apache-harmony-jdwp-tests-host_intermediates/javalib.jar \
        org.apache.harmony.jpda.tests.jdwp.ClassType.InvokeMethodTest
Test: Manual Vogar
Bug: 62821960
Change-Id: I7d6499470493d6ab2ea7c948545b6b7a3fab35bc