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/909-attach-agent/expected.txt b/test/909-attach-agent/expected.txt
new file mode 100644
index 0000000..eacc595
--- /dev/null
+++ b/test/909-attach-agent/expected.txt
@@ -0,0 +1,3 @@
+Hello, world!
+Attached Agent for test 909-attach-agent
+Goodbye!