[tp]: Fix runnable state after process fork

We already parsed the task_newtask ftrace events which tell us when a
new thread is created including process forks. Unfortunately, in the
process fork case, we didn't actually open a runnable state.

Fixed by simply opening a runnable state in the process fork case.

Test: tools/diff_test_trace_processor.py out/android/trace_processor_shell
Test: ./out/android/perfetto_unittests
Bug: 287930911
Change-Id: I991a3685a8124d90617b9ea1a22d390428f3cf5d