Fix JDWP Virtualmachine.Resume command

We used to call ThreadList::UndoDebuggerSuspensions but this is not
correct. Indeed Virtualmachine.Resume command should be performed
as many times as all threads have been suspended (either by the
Virtualmachine.Suspend command or the application of suspend policy
ALL for event).

This CL fixes this issue by adding ThreadList::ResumeAllForDebugger.
It decrements suspend count and debug suspend count by 1 (globally
and for each thread).

It also handles the case where Virtualmachine.Resume is called while
threads have not been suspended (global debug suspend count is 0). In
this case we just print a warning and do nothing.

Bug: 17974047
Change-Id: I25af22b1936760292d2a8c286e0867f2ae864108
diff --git a/runtime/thread_list.h b/runtime/thread_list.h
index 9f47f9f..a7f2c53 100644
--- a/runtime/thread_list.h
+++ b/runtime/thread_list.h
@@ -105,6 +105,11 @@
   void SuspendSelfForDebugger()
       LOCKS_EXCLUDED(Locks::thread_suspend_count_lock_);
 
+  // Resume all threads
+  void ResumeAllForDebugger()
+      LOCKS_EXCLUDED(Locks::thread_list_lock_,
+                     Locks::thread_suspend_count_lock_);
+
   void UndoDebuggerSuspensions()
       LOCKS_EXCLUDED(Locks::thread_list_lock_,
                      Locks::thread_suspend_count_lock_);