Move SetMonitorEnterObject outside of blocked thread state change.

Race condition:
Thread is suspended in monitor kBlocked, GC decides to run the
checkpoint on it. The GC sees that the object is non null, and goes
to mark it, but then the thread does SetMonitorObject(nullptr).
Which causes a null object to be marked.

Change-Id: Ie8a5074112947ec07d01ccb813ca2c1bb9ac7066