Fix for potential moving GC bugs around proxy class.

- Handlerize proxy_class which is live across multiple allocation
  points in ClassLinker::CreateProxyClass().

- In ClassLinker::CreateProxyClass(), insert a proxy class into the
  class table before creating ArtFields for it (and update it later in
  LinkClass()) because the field roots (ArtField::declaring_class_)
  won't be updated by GC unless the class is in the class table. If GC
  happens before they are updated by FixupTemporaryDeclaringClass()
  from LinkClass(), FixupTemporaryDeclaringClass() may not update the
  field roots correctly because the old class may already be moved but
  the fields roots may not. Reduce a window of time where the fields
  roots could be stale.

- In ClassLinker::LinkClass(), directly wrap a new class in a handle
  to avoid a window of time where new_class may be potentially stale.

- Print more diagnostic info about the holder of the field upon a mark
  sweep invalid ref crash.

- Add an additional sanity check in Field::GetArtField().

Bug: 20557050

Change-Id: I9ad32d304922da96b7e1fad262d97de21cbac776