Revert^2 "Maintain separate niceness and priority"

PS1 is identical to ag/34939844

PS2 Fixes an incorrect expectation in 051-thread, and an incorrect
    (obsolete) function invocation in runtime_callbacks_test.
    Also adds minor testing improvements.

Flag: EXEMPT bugfix
Bug: 417492941
Test: testrunner.py --host -b --64 -t 051-thread
(Reverts ag/35050319)

Change-Id: I5c942da7ae1be0c1743e9ef9bafa5c4578c36e5c