Add the missing link between compiled code and the debugger.

When a debugger connects and disconnects, we now let compiled code know that we
need to be kept informed about what's going on.

Also fix a threading bug when threads exit with a debugger attached.

Also some minor tidying, mostly involving naming.

Change-Id: Iba0e8b9d192ac76ba1cd29a8b1e6d94f6f20dea8
diff --git a/src/jdwp/jdwp_main.cc b/src/jdwp/jdwp_main.cc
index 8fa650a..ad2281f 100644
--- a/src/jdwp/jdwp_main.cc
+++ b/src/jdwp/jdwp_main.cc
@@ -414,7 +414,7 @@
  * processing a debugger request.
  */
 int64_t JdwpState::LastDebuggerActivity() {
-  if (!Dbg::IsDebuggerConnected()) {
+  if (!Dbg::IsDebuggerActive()) {
     LOG(DEBUG) << "no active debugger";
     return -1;
   }