Fix utid in critical path stack

_thread_executing_span_critical_path_stack concats the thread states
and slice info on the thread itself with the thread states and slice
info in the critical path of the thread. The 'self' utid was incorrect.

Note that the UI always looked correct, because the thread_name and
process_name columns were not broken. So even though the utid for the
thread_state was broken, it was never displayed in the debug track.

Fixed and added a more useful test that outputs one entire stack in time.

Test: tools/diff_test_trace_processor.py out/android/trace_processor_shell --name-filter '.*thread_executing_span.*'
Change-Id: I3dd3da33de96072564be0abfa8c3bedadaf43379