ART: Fix forbidden thread state change in interpreter

While loading a type for assignability, it might happen that it's
not available yet locally and must be resolved. Formerly, we
disallowed a state change to ensure no GC taking place while a new
shadow frame has not been pushed on the stack yet.

As a fix, allow a "shadow frame under construction" in the thread,
which is visited during GC.

Change-Id: I973487a46b0e9e21fd6d49099d713b58f06d3b45