Fix system_weak_test

A recent change made the SS collector send the Broadcast event. This
caused the system_weak_test to fail when run with this collector.

Test: ./art/test/testrunner/run_build_test_target.py -j30 art-gtest-debug-gc
Bug: 76205593
Change-Id: I8780492240453f51e40808241ed63c455485834e
1 file changed