JDWP: allocate DebugInvokeReq only when requested

JDWP: allocate DebugInvokeReq only when requested

Only allocates thread-local DebugInvokeReq when the debugger requests
a thread to invoke a method. The JDWP thread allocates that structure
then attaches it to the target thread. When the thread is resumed, it
executes the method. Once the invocation completes, the thread
detaches the DebugInvokeReq, signals the JDWP thread then suspends.
Finally, the JDWP thread wakes up, prepares the reply with the invoke
result (or exception) and deallocates the DebugInvokeReq.

Also ensures GC safety for object returned by the invoke. We add the
object to the JDWP object registry right after the invoke. We now
reference that object with a JDWP ObjectID instead of an Object* in
the DebugInvokeReq struct. This prevent from accessing a stale
reference if the GC runs and moves the Object*.

This CL includes the following changes:
- Move former DebugInvokeReq::ready flag to
  Thread::tls_32bit_sized_values::ready_for_debug_invoke. It's needed
  to know whether a thread has been suspended by an event, thus ready
  to invoke a method from the debugger.
- Remove DebugInvokeReq::invoke_needed: we now test if we attached a
  DebugInvokeReq* to the thread.
- Rename misleading FinishMethod function to RequestMethod.

Bug: 19142632
Bug: 18166750
Change-Id: I351fb4eb94bfe69fcafb544d21d55ff35a033000