JDWP: do not report start/end events for thread without peer

To report start/end thread events, we need the JDWP id of the thread.
To create a JDWP id, we need its java.lang.Thread peer. In the case
there is no Java peer, we can't report the event.

Follow-up https://android-review.googlesource.com/118548 where the
runtime attaches the current thread without Java peer when shutting
down.

Change-Id: Icc2d50f3d2eedd1e0e3c5a21affd2261aaf7a862