Revert^2 "Prevent agents from delaying system daemon threads"

It looks like there might be a race between DetachCurrentThread and
runtime shutdown that the original CL exposed with test 1919. I'm not
sure why this test exposed this issue, possibly it's due to the
removal of the Daemon events making the test faster, thus more likely
to get to runtime-shutdown without the testing thread having fully
detached.

This reverts commit c08c1071.

Reason for revert: (Hopefully) made test 1919 resistant to the flake
                   it was experiencing.
Test: ./test.py --host --ndebug -j80

Bug: 123696564
Bug: 124284724

Change-Id: I2e58a71aa174f6b89e57c31378b0fe5bb41a8844