Enable agent attaching during live phase

This is the ART part, the plumbing from VMDebug_attachAgent() to
actually loading the agent into the runtime.

Test: m test-art-host

Bug: 31682382

Change-Id: I3ccc67aa050c1f78278882128983686ed44ddec2
diff --git a/test/Android.run-test.mk b/test/Android.run-test.mk
index d962472..9b456c1 100644
--- a/test/Android.run-test.mk
+++ b/test/Android.run-test.mk
@@ -277,6 +277,7 @@
   906-iterate-heap \
   907-get-loaded-classes \
   908-gc-start-finish \
+  909-attach-agent \
 
 ifneq (,$(filter target,$(TARGET_TYPES)))
   ART_TEST_KNOWN_BROKEN += $(call all-run-test-names,target,$(RUN_TYPES),$(PREBUILD_TYPES), \