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/runtime/debugger.cc b/runtime/debugger.cc
index e2b8f51..1da888e 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -551,6 +551,10 @@
   gJdwpAllowed = allowed;
 }
 
+bool Dbg::IsJdwpAllowed() {
+  return gJdwpAllowed;
+}
+
 DebugInvokeReq* Dbg::GetInvokeReq() {
   return Thread::Current()->GetInvokeReq();
 }