Don't write to inferior_ptid in remote.c

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_target::remote_notice_new_inferior): Use
	switch_to_thread instead of writing to inferior_ptid directly.
	(remote_target::add_current_inferior_and_thread): Use
	switch_to_no_thread instead of writing to inferior_ptid directly.
	(extended_remote_target::attach): Use switch_to_inferior_no_thread
	and switch_to_thread instead of using set_current_inferior or
	writing to inferior_ptid directly.