Decrement suspend count if thread is shutting down

Prevents deadlock caused by incrementing suspend count in
SuspendThreadByPeer, then getting a cleared nativePeer field. This
resulted in us not decrementing the suspend count which caused a
deadlock in WaitForOtherNonDaemonThreadsToExit.

Bug: 18739541
Change-Id: I4a63f1823993a0f99f32025cd479072be49ba8d5
Merge "Decrement suspend count if thread is shutting down"