Fix crash in VirtualMachine.AllThreads JDWP command

When collecting all the started threads, we may encounter a thread in the
process of being started from Java code (Thread.start) in the threads list. The
crash occurs when we attempt to access the java.lang.Thread peer (calling
Thread::GetPeer) but the JNI global reference to it (Thread::jpeer) has not
been destroyed yet (which is verified with a check). This only happens when the
thread is starting up.

We now check the thread finished starting up before accessing the Java peer.
This also prevents from returning non-started threads in the reply.

Bug: 16660101

Change-Id: I9e445e5da3a6951143d6c3c9a0d4f136398fde2f