Update return value after method exit hook callback

MethodExitHook could potentially cause a GC and we need to update
the return value since GC could move a reference.

Test: art/test.py
Bug: 208587492
Change-Id: Id734a08dd5586c88ac467aa88013f9616cbe4203
(cherry picked from commit 5b7968882e7b0299390592ab79b7ed0eac68cbe1)
Merged-In: Id734a08dd5586c88ac467aa88013f9616cbe4203