Fix JNI for warning about failure to unlink DeathRecipients

We now (a) use the right Class getName() method, and (b) look it
up once at setup time rather than doing that lookup every time we
want to emit the warning.  Verified to work properly and no longer
crash or throw or otherwise complain.

Change-Id: If0767f8845588ba7f34bac21474f4e2ad5c111d6