Minor fixes to CheckJNI.

Minor fixes to CheckJNI.

We weren't always switching to "running" mode before playing with
objects.  Now checkObject() always changes modes, and things that called
checkObject while in "running" mode now do so after switching back to
"native" (which shouldn't matter now, but I don't want to require nested
switching to work if I don't have to).

Vaguely related to internal bug 1943379, since I think we want to do
some updates to CheckJNI there.