perf evsel: Rename get_states() to parse_task_states() and make it public

Since get_states() assumes the existence of libtraceevent, so move
to where it should belong, i.e, util/trace-event-parse.c, and also
rename it to parse_task_states().

Leave evsel_getstate() untouched as it fits well in the evsel
category.

Also make some necessary tweaks for python support, and get it
verified with: perf test python.

Signed-off-by: Ze Gao <zegao@tencent.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240123070210.1669843-2-zegao@tencent.com