Fixed debugger for threads attached from JNI

Dbg::PostThreadStart was not invoked in Thread::Attach,
thus when attaching existing native thread to JNI, JDWP
events from that thread are not handled correctly by the
debugger, because the thread keeps being unannounced.

The issue was reproduced on Eclipse 4.3 and 4.4.

Bug: 18030371
(cherry picked from commit dfafeef1)

Change-Id: Ia91dcf1aa98913aea2742c90b3e21b10d2cb36d4
Signed-off-by: Daniel Mihalyi <daniel.mihalyi@mattakis.com>