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.bp b/test/Android.bp
index aaa1343..be1864c 100644
--- a/test/Android.bp
+++ b/test/Android.bp
@@ -252,6 +252,7 @@
         "906-iterate-heap/iterate_heap.cc",
         "907-get-loaded-classes/get_loaded_classes.cc",
         "908-gc-start-finish/gc_callbacks.cc",
+        "909-attach-agent/attach.cc",
     ],
     shared_libs: [
         "libbase",