JavaBBinder::onTransact - log if env null

This can happen if ProcessState::self()->joinThreadPool() is called
from a thread that is started manually, for instance.

Fatal error is kept (would be SIGSEGV). We don't attach to the JVM
automatically here because it's a resource-intensive operation to
do automatically.

Bug: 225134223
Test: N/A
Change-Id: Ie75b3ee7591c7c07f79c567866f0d26570cf1a4c