ART: Fix and extend test 904

Fix using the wrong thread when unregistering. The inner thread
should not use Thread.currentThread (copy-n-paste mistake).

Add test for registering in OnLoad.

Add test for keeping registered on exit.

Bug: 31684277
Bug: 31684920
Test: m test-art-host-run-test-904-object-allocation
Change-Id: If0d5eeeb864684ab42eeaf62698d1e639f2071d6