Store java_lang_Object_ in the flip callback

There was a race where the GC thread would not have is_marking =
true, and call
WellKnownClasses::ToClass(WellKnownClasses::java_lang_Object). This
meant that the returned class was maybe in the from-space for the
no image case. The fix was to move this decoding into the flip
callback since this callback is called before flipping any thread
roots.

Bug: 37531237
Bug: 12687968
Test: test-art-host

Change-Id: I9a09249e9c6ea2b3b124e957a9e4b61017869306